[Sentence] Fix infinite loop bug

[pommedeterresautee]

Sometimes, start_pos in Token is set to None.
This is for instance the case when loading a Corpus in columnar format.

In those cases while t.start_pos != pos from to_original_text method is always True.
This fix manages this case.

Another solution would be to just throw an Exception. Let me know what you prefer. In this case it may be good to forbid to have start_pos being None in any case and to rewrite Corpus class. May be to put in another PR

[alanakbik]

👍

[alanakbik]

thanks for fixing this!