fix a crasher in SurfaceTexture::updateTexImage()

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