Fixes: 630/clusterfuzz-testcase-6608718928019456

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 956472a3)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>