[woff2_dec] fix test for UintBase128 overflow

Hello,

I believe there is an involuntary typo in the "woff2_dec.cc" (line 110), inside `ReadBase128`, where it tests if the UintBase128-encoded value is about to overflow.
If I understand this correctly, the purpose of this check is to avoid that the resulting value exceeds 2**32-1.
Is that correct?

All best,

Cosimo