ImagerReader: Return the buffer to queue if it fails to lock

Return the buffer to the queue when failing to lock it with the
error ACQUIRE_NO_BUFFERS, which acquireNextImage of
ImageReader will return null. Otherwise, there is no chance to return
the buffer back later since the caller can't get the new SurfaceImage
object.

Change-Id: I774e91d52f3e10c8d2dd88d835505da79086c5b2