Fix bug with merging diffs with null options

A diff that is created with a NULL options parameter could result
in a NULL prefix string, but diff merge was unconditionally
strdup'ing it.  I added a test to replicate the issue and then a
new method that does the right thing with NULL values.

3 files changed, 14 insertions, 2 deletions

diff --git a/src/diff.c b/src/diff.c
index 4fea894f8..09f319e6e 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -814,9 +814,9 @@ int git_diff_merge(
 
 		/* prefix strings also come from old pool, so recreate those.*/
 		onto->opts.old_prefix =
-			git_pool_strdup(&onto->pool, onto->opts.old_prefix);
+			git_pool_strdup_safe(&onto->pool, onto->opts.old_prefix);
 		onto->opts.new_prefix =
-			git_pool_strdup(&onto->pool, onto->opts.new_prefix);
+			git_pool_strdup_safe(&onto->pool, onto->opts.new_prefix);
 	}
 
 	git_vector_foreach(&onto_new, i, delta)
diff --git a/src/pool.c b/src/pool.c
index 63bf09cee..64b5c6b00 100644
--- a/src/pool.c
+++ b/src/pool.c
@@ -206,6 +206,11 @@ char *git_pool_strdup(git_pool *pool, const char *str)
 	return git_pool_strndup(pool, str, strlen(str));
 }
 
+char *git_pool_strdup_safe(git_pool *pool, const char *str)
+{
+	return str ? git_pool_strdup(pool, str) : NULL;
+}
+
 char *git_pool_strcat(git_pool *pool, const char *a, const char *b)
 {
 	void *ptr;
diff --git a/src/pool.h b/src/pool.h
index 54a2861ed..05d339244 100644
--- a/src/pool.h
+++ b/src/pool.h
@@ -90,6 +90,13 @@ extern char *git_pool_strndup(git_pool *pool, const char *str, size_t n);
 extern char *git_pool_strdup(git_pool *pool, const char *str);
 
 /**
+ * Allocate space and duplicate a string into it, NULL is no error.
+ *
+ * This is allowed only for pools with item_size == sizeof(char)
+ */
+extern char *git_pool_strdup_safe(git_pool *pool, const char *str);
+
+/**
  * Allocate space for the concatenation of two strings.
  *
  * This is allowed only for pools with item_size == sizeof(char)