v4l2: fix race condition

v4l2: fix race condition

The size of the buffer would be zero'd out in gst_v4l2_buffer_finalize()
after the buffer is qbuf'd or pushed onto the queue of available buffers..
leaving a race condition where the thread waiting for the buffer could awake
and set back a valid size before the finalizing thread zeros out the length.
This would result that the newly allocated buffer has length of zero.