Fixes: division by zero
Fixes: 21677/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DSD_MSBF_fuzzer-5712547983654912
Fixes: 21751/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DSD_LSBF_fuzzer-5197097180856320

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