don't return the current buffer from dequeueBuffer

we were not reseting mCurrentTexture in some situations
which in turn caused dequeueBuffers() return a
"FREE" buffer that was also current.
Very often it was harmless, but it created a race with
updateTexImage() which could cause the following
queueBuffers() to fail.

Bug: 5156325
Change-Id: If15a31dc869117543d220d6e5562c57116cbabdb