avcodec/snowdec: Fix off by 1 error

Fixes: runtime error: index 4 out of bounds for type 'int8_t [4]'
Fixes: 3023/clusterfuzz-testcase-minimized-6421736130084864

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