avcodec/s302m: Fix left shift of 8 by 28 places cannot be represented in type 'int'

Fixes: 1395/clusterfuzz-testcase-minimized-5330939741732864

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