Fixes: 709/clusterfuzz-testcase-4789836449841152

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 d757ddba)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>