avcodec/h264_mp4toannexb_bsf: add a case when only SPS/PPS is in the stream.

When only SPS or PPS is present in the stream, copy the missing one from
AVCC before insertion to the output stream.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/h264_mp4toannexb_bsf: use the given padding in h264_extradata_to_annexb().

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