TNTLexer crashes with input '0'

[kurtmckee]

While working on #1425 I used Hypothesis to help me identify JSON lexing issues.

Hypothesis immediately reported a crash bug in the TNT lexer. It crashes with an IndexError if you pass 0 as the input string:

>>> import pygments.lexers
>>> list(pygments.lexers.TNTLexer().get_tokens('0'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\Kurt\Documents\dev\pygments\pygments\lexer.py", line 188, in streamer
    for _, t, v in self.get_tokens_unprocessed(text):
  File "C:\Users\Kurt\Documents\dev\pygments\pygments\lexers\tnt.py", line 215, in get_tokens_unprocessed
    start = end = self.formula(start, text)
  File "C:\Users\Kurt\Documents\dev\pygments\pygments\lexers\tnt.py", line 107, in formula
    if text[start] in '[]':  # fantasy push or pop
IndexError: string index out of range

[kurtmckee]

@Kenny2github and @lonetwin, would you review this when you have an opportunity? Thanks!

Related: #1414

[Kenny2github]

0 is not valid as a string in TNT, though it shouldn't crash. Are error tokens acceptable in the output?

[kurtmckee]

Yes, error tokens are acceptable.