Fixes: assertion failure
Fixes: 38332/clusterfuzz-testcase-minimized-ffmpeg_dem_DHAV_fuzzer-4522405595316224

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>