[WIP][ticket/12413] Fatal Error for feed.php?mode=forums

https://tracker.phpbb.com/browse/PHPBB3-12413
http://area51.phpbb.com/phpBB/viewtopic.php?f=81&t=45475

Signed-off-by: Tristan Darricau <github@darricau.eu>

PHPBB3-12413