Fix undefined var when posting new top-level comment

[neapsix]

Hello! Here's a small patch for you to consider.

With 0.24, when I post a comment that's not a reply, if the recent_changes module is enabled, I see the following PHP warning and the automatic redirect to my posted comment doesn't work.

Warning: Undefined variable $comment_thread in /usr/local/www/wiki/index.php on line 3956

This PR fixes the issue. I've tested it on FreeBSD with Apache and PHP 8.0, 8.1, and 8.2.

Thank you for all your work on Pepperminty Wiki—what a fabulous bit of software!

I agree to release this contribution under the Mozilla Public License 2.0.