avcodec/vp3dsp: Fix multiple signed integer overflow: 46341 * 47523 cannot be represented in type 'int'

Fixes: 664/clusterfuzz-testcase-4917047475568640

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