avcodec/v4l2_m2m_dec: remove redundant packet and fix double free
v4l2_receive_frame() uses two packets s->buf_pkt and avpkt. If avpkt
cannot be enqueued, the packet is buffered in s->buf_pkt and enqueued in
the next call. Currently the ownership transfer between the two packets
is not properly handled. A double free occurs if
ff_v4l2_context_enqueue_packet() returns EAGAIN and v4l2_try_start
returns EINVAL.
In fact, having two AVPackets is not needed and everything can be
handled by s->buf_pkt.
This commit removes the local avpkt from v4l2_receive_frame(), meaning
that the ownership transfer doesn't need to be handled and the double
free is fixed.