Fixes: 617/clusterfuzz-testcase-6413875723370496

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 c11d3634)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>