avcodec/snowdec: Fix runtime error: signed integer overflow: 1404 * 8388608 cannot be represented in type 'int'

Fixes: 2004/clusterfuzz-testcase-minimized-5533262866808832

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