[pvr] fix marking recordings as watched or unwatched

This PR fixes there distinct bugs:

* when a recording was marked as watched/unwatched. This is because the change was done on a copy of the actual recording, not the correct instance.

* after marking something as watched/unwatched, the selected item didn't change to the next one in the list (as is done in other windows)

* a folder containing only watched items wasn't marked as watched until the folder had been entered once

Fixes http://trac.xbmc.org/ticket/15118
missing null check