Small repair of Warning (and showing category of event) in show event shared from another user.

Warning: Invalid argument supplied for foreach() in /var/www/owncloud/apps/calendar/templates/part.showevent.php on line 26

Issue: https://github.com/owncloud/apps/issues/678
Repaired santization
Merge pull request #680 from honza889/master

Small repair of foreach in calendar/templates/part.showevent.php