For a corrupted stream, the value of nalu_len read from the extradata is not reliable.
We need to perform additional checks

Fixes: fuzzer timeout
Fixes: 65253/clusterfuzz-testcase-minimized-ffmpeg_BSF_VVC_MP4TOANNEXB_fuzzer-4972412487467008

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