Fixes: integer overflow
Fixes: 22285/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SNOW_fuzzer-5682428762128384

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 38fbf33c7255b503453052c32ab5ae4fb151b29e)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>