avcodec/snowdec: Fix integer overflow before htaps check

Fixes: runtime error: signed integer overflow: -1094995529 * 2 cannot be represented in type 'int'
Fixes: 4828/clusterfuzz-testcase-minimized-5100849937252352

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