BnGraphicBufferProducer: Add null fence checks

Add null fence checks to BnGraphicBufferProducer::onTransact
DEQUEUE_BUFFER and GET_LAST_QUEUED_BUFFER cases. If we do find a null
fence, set to Fence::NO_FENCE and log an error.
This will avoid a crash. There still may be a synchronization issue
with NO_FENCE, but a glitch is favorable to a crash and the extra error
logging will help isolate any null fence issues going forward.

Fix: 124395884
Test: builds
Change-Id: I458aadf183e638dea04a9457cc7df5579a7e8046