Fix infinite loop when mThreadingCache is 'corrupted'

Summary: For some reason i have a mThreadingCache that if you obey with the eyes closed creates loops in the threading and thus kmail gets stuck when trying to show that folder

Test Plan: Folder no longer gets stuck in an infinite loop

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: vkrause, mlaurent, winterz, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D21737