avviddec: Fix crash with FFmpeg n6.0
FFmpeg version 6.0 has an AV_CODEC_FLAG_COPY_OPAQUE related bug.
See https://github.com/FFmpeg/FFmpeg/commit/a3bf63c26e6197a9b6e490538667368530a9dcd3
FFmpeg version 6.0 has an AV_CODEC_FLAG_COPY_OPAQUE related bug.
See https://github.com/FFmpeg/FFmpeg/commit/a3bf63c26e6197a9b6e490538667368530a9dcd3