avcodec/avcodec: Limit the number of side data elements per packet

Fixes: 1293/clusterfuzz-testcase-minimized-6054752074858496

See: [FFmpeg-devel] [PATCH] avcodec/avcodec: Limit the number of side data elements per packet

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