Fix leaks in purple markup

Fix leaks when running the markup tests. These are all from leaking `tags`:
```
24 bytes in 1 blocks are definitely lost in loss record 101 of 251
   at 0x4848464: calloc (vg_replace_malloc.c:1340)
   by 0x49F75F0: g_malloc0 (gmem.c:163)
   by 0x48E563C: purple_markup_html_to_xhtml (purplemarkup.c:437)
   by 0x401214: test_purple_markup_html_to_xhtml (test_markup.c:213)
   by 0x4A1CC7D: UnknownInlinedFun (gtestutils.c:2933)
   by 0x4A1CC7D: g_test_run_suite_internal (gtestutils.c:3021)
   by 0x4A1C9E4: g_test_run_suite_internal (gtestutils.c:3038)
   by 0x4A1C9E4: g_test_run_suite_internal (gtestutils.c:3038)
   by 0x4A1D181: g_test_run_suite (gtestutils.c:3115)
   by 0x4A156EC: UnknownInlinedFun (gtestutils.c:2234)
   by 0x4A156EC: g_test_run (gtestutils.c:2221)
   by 0x401575: main (test_markup.c:281)

24 bytes in 1 blocks are definitely lost in loss record 102 of 251
   at 0x4848464: calloc (vg_replace_malloc.c:1340)
   by 0x49F75F0: g_malloc0 (gmem.c:163)
   by 0x48E0319: purple_markup_html_to_xhtml (purplemarkup.c:260)
   by 0x401214: test_purple_markup_html_to_xhtml (test_markup.c:213)
   by 0x4A1CC7D: UnknownInlinedFun (gtestutils.c:2933)
   by 0x4A1CC7D: g_test_run_suite_internal (gtestutils.c:3021)
   by 0x4A1C9E4: g_test_run_suite_internal (gtestutils.c:3038)
   by 0x4A1C9E4: g_test_run_suite_internal (gtestutils.c:3038)
   by 0x4A1D181: g_test_run_suite (gtestutils.c:3115)
   by 0x4A156EC: UnknownInlinedFun (gtestutils.c:2234)
   by 0x4A156EC: g_test_run (gtestutils.c:2221)
   by 0x401575: main (test_markup.c:281)

24 bytes in 1 blocks are definitely lost in loss record 103 of 251
   at 0x4848464: calloc (vg_replace_malloc.c:1340)
   by 0x49F75F0: g_malloc0 (gmem.c:163)
   by 0x48E07B7: purple_markup_html_to_xhtml (purplemarkup.c:261)
   by 0x401214: test_purple_markup_html_to_xhtml (test_markup.c:213)
   by 0x4A1CC7D: UnknownInlinedFun (gtestutils.c:2933)
   by 0x4A1CC7D: g_test_run_suite_internal (gtestutils.c:3021)
   by 0x4A1C9E4: g_test_run_suite_internal (gtestutils.c:3038)
   by 0x4A1C9E4: g_test_run_suite_internal (gtestutils.c:3038)
   by 0x4A1D181: g_test_run_suite (gtestutils.c:3115)
   by 0x4A156EC: UnknownInlinedFun (gtestutils.c:2234)
   by 0x4A156EC: g_test_run (gtestutils.c:2221)
   by 0x401575: main (test_markup.c:281)

24 bytes in 1 blocks are definitely lost in loss record 104 of 251
   at 0x4848464: calloc (vg_replace_malloc.c:1340)
   by 0x49F75F0: g_malloc0 (gmem.c:163)
   by 0x48E0C55: purple_markup_html_to_xhtml (purplemarkup.c:262)
   by 0x401214: test_purple_markup_html_to_xhtml (test_markup.c:213)
   by 0x4A1CC7D: UnknownInlinedFun (gtestutils.c:2933)
   by 0x4A1CC7D: g_test_run_suite_internal (gtestutils.c:3021)
   by 0x4A1C9E4: g_test_run_suite_internal (gtestutils.c:3038)
   by 0x4A1C9E4: g_test_run_suite_internal (gtestutils.c:3038)
   by 0x4A1D181: g_test_run_suite (gtestutils.c:3115)
   by 0x4A156EC: UnknownInlinedFun (gtestutils.c:2234)
   by 0x4A156EC: g_test_run (gtestutils.c:2221)
   by 0x401575: main (test_markup.c:281)

24 bytes in 1 blocks are definitely lost in loss record 105 of 251
   at 0x4848464: calloc (vg_replace_malloc.c:1340)
   by 0x49F75F0: g_malloc0 (gmem.c:163)
   by 0x48E10F3: purple_markup_html_to_xhtml (purplemarkup.c:263)
   by 0x401214: test_purple_markup_html_to_xhtml (test_markup.c:213)
   by 0x4A1CC7D: UnknownInlinedFun (gtestutils.c:2933)
   by 0x4A1CC7D: g_test_run_suite_internal (gtestutils.c:3021)
   by 0x4A1C9E4: g_test_run_suite_internal (gtestutils.c:3038)
   by 0x4A1C9E4: g_test_run_suite_internal (gtestutils.c:3038)
   by 0x4A1D181: g_test_run_suite (gtestutils.c:3115)
   by 0x4A156EC: UnknownInlinedFun (gtestutils.c:2234)
   by 0x4A156EC: g_test_run (gtestutils.c:2221)
   by 0x401575: main (test_markup.c:281)
```
and these from not freeing the result from `purple_unescape_html`:
```
256 bytes in 2 blocks are definitely lost in loss record 241 of 251
   at 0x484378A: malloc (vg_replace_malloc.c:392)
   by 0x484870B: realloc (vg_replace_malloc.c:1451)
   by 0x49F771F: g_realloc (gmem.c:201)
   by 0x4A11343: g_string_maybe_expand (gstring.c:92)
   by 0x4A113BF: g_string_sized_new (gstring.c:116)
   by 0x48E7F74: purple_unescape_html (purplemarkup.c:1101)
   by 0x48DEBBA: purple_markup_html_to_xhtml (purplemarkup.c:224)
   by 0x401214: test_purple_markup_html_to_xhtml (test_markup.c:213)
   by 0x4A1CC7D: UnknownInlinedFun (gtestutils.c:2933)
   by 0x4A1CC7D: g_test_run_suite_internal (gtestutils.c:3021)
   by 0x4A1C9E4: g_test_run_suite_internal (gtestutils.c:3038)
   by 0x4A1C9E4: g_test_run_suite_internal (gtestutils.c:3038)
   by 0x4A1D181: g_test_run_suite (gtestutils.c:3115)

256 bytes in 2 blocks are definitely lost in loss record 242 of 251
   at 0x484378A: malloc (vg_replace_malloc.c:392)
   by 0x484870B: realloc (vg_replace_malloc.c:1451)
   by 0x49F771F: g_realloc (gmem.c:201)
   by 0x4A11343: g_string_maybe_expand (gstring.c:92)
   by 0x4A113BF: g_string_sized_new (gstring.c:116)
   by 0x48E7F74: purple_unescape_html (purplemarkup.c:1101)
   by 0x48E534D: purple_markup_html_to_xhtml (purplemarkup.c:397)
   by 0x401214: test_purple_markup_html_to_xhtml (test_markup.c:213)
   by 0x4A1CC7D: UnknownInlinedFun (gtestutils.c:2933)
   by 0x4A1CC7D: g_test_run_suite_internal (gtestutils.c:3021)
   by 0x4A1C9E4: g_test_run_suite_internal (gtestutils.c:3038)
   by 0x4A1C9E4: g_test_run_suite_internal (gtestutils.c:3038)
   by 0x4A1D181: g_test_run_suite (gtestutils.c:3115)
```

Also modified the test to check not passing the optional arguments.

Testing Done:
Compiled and ran tests in valgrind and leaks were gone.

Reviewed at https://reviews.imfreedom.org/r/2379/

2 files changed, 24 insertions, 5 deletions

diff --git a/libpurple/purplemarkup.c b/libpurple/purplemarkup.c
index 45cb21704e..528c20805a 100644
--- a/libpurple/purplemarkup.c
+++ b/libpurple/purplemarkup.c
@@ -221,7 +221,12 @@ purple_markup_html_to_xhtml(const char *html, char **xhtml_out,
 							if (cdata && url &&
 									(!g_string_equal(cdata, url) && (g_ascii_strncasecmp(url->str, "mailto:", 7) != 0 ||
 									                                 g_utf8_collate(url->str + 7, cdata->str) != 0)))
-									g_string_append_printf(plain, " <%s>", g_strstrip(purple_unescape_html(url->str)));
+							{
+								char *unescaped = purple_unescape_html(url->str);
+								g_string_append_printf(plain, " <%s>",
+								                       g_strstrip(unescaped));
+								g_free(unescaped);
+							}
 							if (cdata) {
 								g_string_free(cdata, TRUE);
 								cdata = NULL;
@@ -393,10 +398,14 @@ purple_markup_html_to_xhtml(const char *html, char **xhtml_out,
 					if(src && xhtml)
 						g_string_append_printf(xhtml, "<img src='%s' alt='%s' />", g_strstrip(src->str), alt ? alt->str : "");
 					if(alt) {
-						if(plain)
-							plain = g_string_append(plain, purple_unescape_html(alt->str));
-						if(!src && xhtml)
+						if(plain) {
+							char *unescaped = purple_unescape_html(alt->str);
+							plain = g_string_append(plain, unescaped);
+							g_free(unescaped);
+						}
+						if(!src && xhtml) {
 							xhtml = g_string_append(xhtml, alt->str);
+						}
 						g_string_free(alt, TRUE);
 					}
 					g_string_free(src, TRUE);
@@ -614,7 +623,7 @@ purple_markup_html_to_xhtml(const char *html, char **xhtml_out,
 				g_string_append_printf(xhtml, "</%s>", pt->dest_tag);
 		}
 	}
-	g_list_free(tags);
+	g_clear_list(&tags, g_free);
 	if(xhtml_out)
 		*xhtml_out = g_string_free(xhtml, FALSE);
 	if(plain_out)
diff --git a/libpurple/tests/test_markup.c b/libpurple/tests/test_markup.c
index a77e9617a6..5a08517ee9 100644
--- a/libpurple/tests/test_markup.c
+++ b/libpurple/tests/test_markup.c
@@ -217,6 +217,16 @@ test_purple_markup_html_to_xhtml(void) {
 
 		g_assert_cmpstr(data[i].plaintext, ==, plaintext);
 		g_free(plaintext);
+
+		/* Check only asking for xhtml works. */
+		purple_markup_html_to_xhtml(data[i].markup, &xhtml, NULL);
+		g_assert_cmpstr(data[i].xhtml, ==, xhtml);
+		g_free(xhtml);
+
+		/* Check only asking for plaintext works. */
+		purple_markup_html_to_xhtml(data[i].markup, NULL, &plaintext);
+		g_assert_cmpstr(data[i].plaintext, ==, plaintext);
+		g_free(plaintext);
 	}
 }