[ticket/12418] Notice displayed for feed.php

https://tracker.phpbb.com/browse/PHPBB3-12418

language/viewtopic.php isn't loaded, so MISSING_INLINE_ATTACHMENT isn't
available.

PHPBB3-12418
Merge remote-tracking branch 'Nicofuma/ticket/12418' into develop-ascraeus

* Nicofuma/ticket/12418:
  [ticket/12418] Notice displayed for feed.php