buffer: do not pass NULL to memcpy() from evbuffer_pullup()

UBSAN reports:

  evbuffer/remove_buffer_with_empty3: ../buffer.c:1443:3: runtime error: null pointer passed as argument 2, which is declared to never be null
      #0 0x7ffff6cd0410 in evbuffer_pullup ../buffer.c:1443
      #1 0x5555556d68b9 in test_evbuffer_remove_buffer_with_empty3 ../test/regress_buffer.c:408
      #2 0x5555557b95ee in testcase_run_bare_ ../test/tinytest.c:173
      #3 0x5555557ba048 in testcase_run_one ../test/tinytest.c:333
      #4 0x5555557bc0f8 in tinytest_main ../test/tinytest.c:527
      #5 0x555555787702 in main ../test/regress_main.c:528
      #6 0x7ffff606c001 in __libc_start_main (/usr/lib/libc.so.6+0x27001)
      #7 0x55555569436d in _start (/src/le/libevent/.cmake-debug/bin/regress+0x14036d)

1 files changed, 31 insertions, 0 deletions

diff --git a/test/regress_buffer.c b/test/regress_buffer.c
index b4c45edc..6d55aead 100644
--- a/test/regress_buffer.c
+++ b/test/regress_buffer.c
@@ -426,6 +426,36 @@ test_evbuffer_remove_buffer_with_empty3(void *ptr)
 }
 
 static void
+test_evbuffer_pullup_with_empty(void *ptr)
+{
+	struct evbuffer *buf = NULL;
+
+	buf = evbuffer_new();
+	evbuffer_add(buf, "foo", 3);
+	evbuffer_add_reference(buf, NULL, 0, NULL, NULL);
+	evbuffer_validate(buf);
+	tt_int_op(evbuffer_get_length(buf), ==, 3);
+	tt_mem_op(evbuffer_pullup(buf, -1), ==, "foo", 3);
+
+	evbuffer_free(buf);
+	buf = evbuffer_new();
+	evbuffer_validate(buf);
+	tt_int_op(evbuffer_get_length(buf), ==, 0);
+	tt_int_op(evbuffer_pullup(buf, -1), ==, NULL);
+
+	evbuffer_free(buf);
+	buf = evbuffer_new();
+	evbuffer_add(buf, "foo", 3);
+	evbuffer_add_reference(buf, NULL, 0, NULL, NULL);
+	evbuffer_validate(buf);
+	tt_mem_op(evbuffer_pullup(buf, 3), ==, "foo", 3);
+
+ end:
+	if (buf)
+		evbuffer_free(buf);
+}
+
+static void
 test_evbuffer_remove_buffer_with_empty_front(void *ptr)
 {
 	struct evbuffer *buf1 = NULL, *buf2 = NULL;
@@ -2810,6 +2840,7 @@ struct testcase_t evbuffer_testcases[] = {
 	{ "add_iovec", test_evbuffer_add_iovec, 0, NULL, NULL},
 	{ "copyout", test_evbuffer_copyout, 0, NULL, NULL},
 	{ "file_segment_add_cleanup_cb", test_evbuffer_file_segment_add_cleanup_cb, 0, NULL, NULL },
+	{ "pullup_with_empty", test_evbuffer_pullup_with_empty, 0, NULL, NULL },
 
 #define ADDFILE_TEST(name, parameters)					\
 	{ name, test_evbuffer_add_file, TT_FORK|TT_NEED_BASE,		\