stash test: free the reference

tests/stash/save.c

@@ -225,6 +225,7 @@ void test_stash_save__untracked_regression(void)
 
 	assert_commit_message_contains("refs/stash", "WIP on master");
 
+	git_reference_free(head);
 	git_buf_free(&untracked_dir);
 }