Avoid infinite loops when fetching the URL from DocumentUrlDB

Summary:
Some users apparently have DBs which contain infinite loops in the idTree
(a parentId pointing to itself or one of its children). This manifests in
either crashes due to exhausted memory and/or the process being stuck
with 100% CPU load.

The problem can only be completely solved by either recreating the DB, or
by cleaning the DB from any problematic records. As in interim solution,
stop the code from crashing.

BUG: 378754
CCBUG: 385846
CCBUG: 391258
CCBUG: 393181

Reviewers: Baloo, michaelh, Frameworks, ngraham

Reviewed By: ngraham

Subscribers: ngraham, Frameworks

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D12335