Fix infinite loop guard in case of duplicate UIDs

Summary:
In this case the prenodeByUid hash will not increase its size (thus not
fulfilling the outer loop condition), but an element is found nevertheless
(thus not triggering the loop guard). Now both use the same condition.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D21518