[ticket/11524] Add another isset() to mitigate "Illegal string offset 'limit'"

... on PHP 5.4 or higher.

PHPBB3-11524
Merge remote-tracking branch 'bantu/ticket/11524' into prep-release-3.0.12

* bantu/ticket/11524:
  [ticket/11524] Add another isset() to mitigate "Illegal string offset 'limit'"