avcodec/snowdec: Fix integer overflow in decode_subband_slice_buffered()

Fixes: runtime error: signed integer overflow: 267 * 8388608 cannot be represented in type 'int'
Fixes: 2743/clusterfuzz-testcase-minimized-5820652076400640

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