Fixes reading freed memory
Fixes: 568/clusterfuzz-testcase-6107186067406848

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 c03029a8)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>