Fixes: 1425/clusterfuzz-testcase-minimized-6295712339853312

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 ed3c9b5b)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>