avformat/bit: check that pkt->size is 10 in write_packet

Ohter packet sizes are not supported by this muxer.

This avoids a null pointer dereference of pkt->data.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
