avcodec/avpacket/av_packet_split_side_data: ensure that side data padding is initialized

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/utils: add some saftey checks to add_metadata_from_side_data()

This fixes potential overreads with crafted files.

Found-by: wm4
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>