we now make sure to drain the buffer queue on disconnect.
this happens only when in synchrnous mode. in async mode
we clear all buffers except the head of the queue.
for extra safety we also catch the null pointer
in updateTexImage (which should never happen) and return
an error.
Bug: 5111008
Change-Id: I5174a6ecbb0de641c6510ef56a611cbb4e9e1f59
5c71575983