Fixes: division by 0
Fixes: 26780/clusterfuzz-testcase-minimized-ffmpeg_dem_ADF_fuzzer-5117945027756032
Fixes: 26998/clusterfuzz-testcase-minimized-ffmpeg_dem_ADF_fuzzer-5119352359354368

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>