Fixed bug where relationship field would return a post_id if the post had been deleted

http://support.advancedcustomfields.com/discussion/5298/relationship-is_object
Signed-off-by: Elliot Condon <e@elliotcondon.com>
Fixed similar issue as previous relationship bug for the post_object field

Signed-off-by: Elliot Condon <e@elliotcondon.com>