crash on very long tokens

[snarfed]

ran into this while working on snarfed/bridgy#723. not blocking me there, but figured it was worth tracking at least.

>>> brevity.shorten('X' * 141)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../brevity.py", line 231, in shorten
    return ''.join(t.content for t in tokens)
TypeError: sequence item 0: expected string or Unicode, NoneType found

tokens at this point is [None, u'\u2026'].

[kylewm]

whoa! weird, thanks