Replace several Q_ASSERTs with proper checks

Summary:
The code has some preconditions on supplied values when interacting
with the DB. Instead of silently corrupting the DB with bogus values
when using non-debug builds, check any provided arguments.

Reviewers: Baloo, Frameworks, ngraham, poboiko, dhaumann

Reviewed By: dhaumann

Subscribers: dhaumann, ngraham, apol, sitter, kde-frameworks-devel, broulik, Frameworks

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D12336