avcodec/vp3dsp: Use unsigned constant to avoid undefined integer overflow in ff_vp3ds...

avcodec/vp3dsp: Use unsigned constant to avoid undefined integer overflow in ff_vp3dsp_set_bounding_values()

Fixes: signed integer overflow: 64 * 33686018 cannot be represented in type 'int'
Fixes: 26911/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_THEORA_fuzzer-4904975073017856
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Peter Ross <pross@xvid.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>