[server] Fix possible crash after deleting an output(device)interface
Review Request #126097 - Created Nov. 17, 2015 and submitted
After deleting an OutputInterface the resources are not necessarily destroyed, so unbind might still be called. The existing code just casted the resource's user data which could then point to invalidated memory. This change verifies that we still have a Private* for the resource. If not, it doesn't have to do any cleanup anyway.