Fix invalid VCALENDAR data in DB!

[rullzer]

As described in #2249

If we have invalid data in the DB this might fail at some point. We should fix the invalid VCALENDAR data properly!

[avanc]

I also got invalid calendar data in my DB. I read through the different pull request (for NC12, backport to NC11, etc). However, I have no idea how I have to do to repair the DB. @LukasReschke , can you give me some hint?