Fixes: NULL pointer dereference
Fixes: 60269/clusterfuzz-testcase-minimized-ffmpeg_BSF_VVC_METADATA_fuzzer-5215449416335360

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