Fixes: 1387/clusterfuzz-testcase-minimized-4802757766676480

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 464c4b86)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>