avcodec/snowdec: Check qbias

Fixes: signed integer overflow: -1094995529 * 131 cannot be represented in type 'int'
Fixes: 1353/clusterfuzz-testcase-minimized-5208180449607680

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