fixed parsing error when input broken comments

[rhiokim]

String.prototyp.charAt do not return Null.
Return char(String) or empty string ""

var css = require('css');
css.parse('/* comments'); // While loop can not stop

[jonathanong]

can you add a test?

[rhiokim]

@jonathanong yep! sure :-)

[jonathanong]

will you be adding a test?

[jonathanKingston]

@rhiokim I only noticed this after I was ready to push the fix... I wasn't trying to step on your toes at all. Thanks for all the great work guys.