transports: smart: fix memory leak when skipping symbolic refs

When we setup the revision walk for negotiating references with a
remote, we iterate over all references, ignoring tags and symbolic
references. While skipping over symbolic references, we forget to free
the looked up reference, resulting in a memory leak when the next
iteration simply overwrites the variable.

Fix that issue by freeing the reference at the beginning of each
iteration and collapsing return paths for error and success.

src/transports/smart_protocol.c

@@ -273,7 +273,7 @@ static int fetch_setup_walk(git_revwalk **out, git_repository *repo)
 	git_revwalk *walk = NULL;
 	git_strarray refs;
 	unsigned int i;
-	git_reference *ref;
+	git_reference *ref = NULL;
 	int error;
 
 	if ((error = git_reference_list(&refs, repo)) < 0)
@@ -285,6 +285,9 @@ static int fetch_setup_walk(git_revwalk **out, git_repository *repo)
 	git_revwalk_sorting(walk, GIT_SORT_TIME);
 
 	for (i = 0; i < refs.count; ++i) {
+		git_reference_free(ref);
+		ref = NULL;
+
 		/* No tags */
 		if (!git__prefixcmp(refs.strings[i], GIT_REFS_TAGS_DIR))
 			continue;
@@ -297,16 +300,13 @@ static int fetch_setup_walk(git_revwalk **out, git_repository *repo)
 
 		if ((error = git_revwalk_push(walk, git_reference_target(ref))) < 0)
 			goto on_error;
-
-		git_reference_free(ref);
 	}
 
-	git_strarray_free(&refs);
 	*out = walk;
-	return 0;
 
 on_error:
-	git_revwalk_free(walk);
+	if (error)
+		git_revwalk_free(walk);
 	git_reference_free(ref);
 	git_strarray_free(&refs);
 	return error;