avcodec/avpacket: Limit iterations in ff_packet_split_and_drop_side_data()

This avoids scaning beyond what a valid packet can contain
Fixes: Timeout
Fixes: 541/clusterfuzz-testcase-610189291657625

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
