Fixes out of array read
Fixes: 1349/clusterfuzz-testcase-minimized-5370707196248064

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 a8ad83b7)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>