Fix WTF in PendingIntentRecord.unregisterCancelListenerLocked()

It's possible that detachCancelListenersLocked() has just been called
when unregisterCancelListenerLocked() is called, so need a null check.

Change-Id: If4b18d713595b1cf1a29ba05a3736892b21931a4
Fixes: 78592185
Test: build
1 file changed