Fix DPLforum b0rkage.

Apparently it was "totally broken", as reported by GreenReaper on #mediawiki: "All DPLForum pages will freeze with that code, which went out on the REL1_23 branch. If you put the assignment back into the while it works again."

Quoth our coding conventions (https://www.mediawiki.org/wiki/Manual:Coding_conventions/PHP):
"Using assignment in a while() clause used to be legitimate, for iteration [...] This is unnecessary in new code; instead use: foreach ( $res as $row )"

This is a follow-up to I24dceed8944d79eaeebf619ac8e449e7d02ece14.

Change-Id: I29248098c6fcd7cafe27f3007e89bb23f31fa0da