lavd/v4l2: copy frames into normally allocated packets whenever there is just one buffer left available

This will avoid the possibility that we dequeue more buffers than we
have obtained from the v4l2 driver.

Fixes ticket #1570

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
v4l2: fix regression that caused ffmpeg to occasionally get stuck

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>