vp10: skip unreachable cat6 token extrabits.

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