More robust event sharing - fixes 142

I got two databases where all shared events disappeared. I cant reproduce how this happened, but with this fix the broken events  get filted and a msg goes into the log. All working events are visible. 

owncloud/apps/calendar/ajax/events.php#33 fails because it gets a incomplete array.

The problem is, that events are oc_shared but not in oc_clndr_objects. 

Hope this helps, and we find a way to reproduce and the real problem.

This fix can go into master, too.