cbs_sei: Detect payload overflows when reading SEI messages

cbs_sei: Detect payload overflows when reading SEI messages

The top-level GetBitContext is sized for the whole NAL unit, so it fails
to detect overflows where a payload continues into the following message.
To fix that, we make a new context on the stack for reading each payload.

Fixes: 29892/clusterfuzz-testcase-minimized-ffmpeg_BSF_H264_REDUNDANT_PPS_fuzzer-6310830956216320
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Tested-by: Michael Niedermayer <michael@niedermayer.cc>