Fixes: 2010/clusterfuzz-testcase-minimized-6209288450080768

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 29808fff)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>