Akonadi: avoid 100% CPU on reading corrupt files.

Summary:
I wonder if the file format changed? I ended up with entityCnt being a very large number,
although the .dat file was very small. In 4 different resources, after migrating from
Leap 42.2 to akonadi master.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: mlaurent, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D3672

(cherry picked from commit 98e72dd81f3b8d5faeb262f765cda9539d1c271f)