Resolves #2688 - Incorrect JSLINT status bar count

Resolves #2688 - Currently jslint will push a `null` value onto `JSLINT.errors` at line [6597](https://github.com/douglascrockford/JSLint/blob/2347ec449b14eb3a4935daeaaa3ef036d42dd96d/jslint.js#L6597). This means an accurate status bar needs to only count non-null jslint warnings. 

This would be more elegant by using `Array.prototype.filter`, but as this is only implemented in JS versions 1.6 onwards I'll prefer the uglier `for` loop version for higher compatibility.