Fixes: out of array access
Fixes: 490576036/clusterfuzz-testcase-minimized-ffmpeg_BSF_EXTRACT_EXTRADATA_fuzzer-4605696279904256
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>