[ticket/12912] Undefined index when adding logs from extensions

Add isset() to forum_id and topic_id.

PHPBB3-12912
Merge pull request #2798 from tmbackoff/ticket/12912

[ticket/12912] Undefined index when adding logs from extensions

* tmbackoff/ticket/12912:
  [ticket/12912] Undefined index when adding logs from extensions
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12912] Undefined index when adding logs from extensions