D12336: Replace several Q_ASSERTs with proper checks

[Stefan Brüns]

bruns added a reviewer: Frameworks.

REPOSITORY
  R293 Baloo

REVISION DETAIL
  https://phabricator.kde.org/D12336

To: bruns, #baloo, michaelh, #frameworks
Cc: #frameworks, ashaposhnikov, michaelh, astippich, spoorun, bruns