Fixes: array end overread
Fixes: 22395/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_BITPACKED_fuzzer-5760940300828672

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