[ticket/12375] Fix broken attachment deletion.

The response is already parsed and when running it through parseJSON returned
null prior to 1.9. Now there's an actual error thrown, so the function is
returning and thus leaving the attachment row intact.

PHPBB3-12375
Merge remote-tracking branch 'prototech/ticket/12375' into develop-ascraeus

* prototech/ticket/12375:
  [ticket/12375] Fix broken attachment deletion.