Fixing a pathological case for slow tokenizers
Author: Narsil
Pull request: yes

# What does this PR do?

Fixes issue found here for slow tokenizers:

https://github.com/huggingface/tokenizers/issues/848

When using arbitrary tokens, a bug could occur where we would seed the new character, even when the lookahead attempted to skip over that part of the text. Thus we could have an extra match that didn't fit.

Other bug was during the lookahead we could undermatch (since we were iterating on new characters before checking termination).

The test added covers both cases.

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Fixes # (issue)


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