Fixes: out of array read
Fixes: 1072/clusterfuzz-testcase-6456688074817536
Fixes: 1398/clusterfuzz-testcase-minimized-4576913622302720

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>