Errors without a line/col throw exception

[postcasio]

Uncaught TypeError: Cannot call method 'tokenAtBufferColumn' of undefined

Similar to the issue with some linters returning line 0, some linters do not return any position information at all for some errors. e.g. csslint has a few rules:

https://github.com/CSSLint/csslint/blob/master/src/rules/important.js
https://github.com/CSSLint/csslint/blob/master/src/rules/font-sizes.js

The error occurs here: https://github.com/AtomLinter/Linter/blob/master/lib/linter.coffee#L190

The calculated rowStart ends up being NaN due to the undefined lineStart and line properties. Changing to:

rowStart = parseInt(match.lineStart ? match.line ? 0) - 1

Seems to fix it but I'm not sure if this breaks anything else.