revwalk: fix memory leak in error handling

This is not implemented and should fail, but it should also not leak. To
allow the memory debugger to find leaks and fix this one we test this.

src/revwalk.c

@@ -203,7 +203,8 @@ int git_revwalk_push_range(git_revwalk *walk, const char *range)
 	if (revspec.flags & GIT_REVPARSE_MERGE_BASE) {
 		/* TODO: support "<commit>...<commit>" */
 		git_error_set(GIT_ERROR_INVALID, "symmetric differences not implemented in revwalk");
-		return GIT_EINVALIDSPEC;
+		error = GIT_EINVALIDSPEC;
+		goto out;
 	}
 
 	if ((error = push_commit(walk, git_object_id(revspec.from), 1, false)))

tests/revwalk/basic.c

@@ -400,6 +400,15 @@ void test_revwalk_basic__push_range(void)
 	cl_git_pass(test_walk_only(_walk, commit_sorting_segment, 2));
 }
 
+void test_revwalk_basic__push_range_merge_base(void)
+{
+	revwalk_basic_setup_walk(NULL);
+
+	git_revwalk_reset(_walk);
+	git_revwalk_sorting(_walk, 0);
+	cl_git_fail_with(GIT_EINVALIDSPEC, git_revwalk_push_range(_walk, "HEAD...HEAD~2"));
+}
+
 void test_revwalk_basic__push_range_no_range(void)
 {
 	revwalk_basic_setup_walk(NULL);