ui-stats.c: set parent pointer to NULL after freeing it

We do this everywhere else, so we should be doing it here as well.

Signed-off-by: John Keeping <john@keeping.me.uk>

ui-stats.c

@@ -246,6 +246,7 @@ static struct string_list collect_stats(struct cgit_period *period)
 		add_commit(&authors, commit, period);
 		free_commit_buffer(commit);
 		free_commit_list(commit->parents);
+		commit->parents = NULL;
 	}
 	return authors;
 }