libavdevice/v4l2: fix invalid access to struct v4l2_buffer

In case we are short of queued buffers, at first v4l2_buffer was enqueued to kernel so it's not owned by
user-space anymore. After that it's timestamp field was read, but it might be overwritten by driver at
that moment. It resulted in invalid timestamp sometimes.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
