Fixes out of array access
Fixes: 1354/clusterfuzz-testcase-minimized-5520132195483648

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit f4ae3cce)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>