avcodec/vp9block: fix runtime error: signed integer overflow: 196675 * 20670 cannot be represented in type 'int'

Fixes: 1710/clusterfuzz-testcase-minimized-4837032931098624

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