Exception during ReDoS detection with malformed expression

[mschwager]

Malformed expressions currently cause Dlint to raise an exception:

$ pipenv run python -m dlint.redos -p '(foo'
Traceback (most recent call last):
...
sre_constants.error: missing ), unterminated subpattern at position 0

Since we'll be running Dlint across many files and don't want it to crash we should handle this error gracefully. We can simply ignore malformed expressions.