vp10: skip unreachable cat6 token extrabits.
We have historically added new bits to cat6 whenever we added a new transform size (or bitdepth, for that matter). However, we have always coded these new bits regardless of the actual transform size, which means that for smaller transforms, we code bits that cannot possibly be set. The coding (quality) impact of this is negligible, but the bigger issue is that this allows creating bitstreams with coefficient values that are nonsensible and can cause int overflows, which then de facto become part of the bitstream spec. By not coding these bits, we remove this possibility. See issue 1065. Change-Id: Ib3186eca2df6a7a15ddc60c8b55af182aadd964d
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