Allow for null message.source

Addresses issue #29. Message source can be null when a Parsing error is
encountered.

Author: solebared

index.js

@@ -182,7 +182,7 @@ module.exports = function(results) {
           var column = message.column || 0;
 
           var arrow = '';
-          var hasSource = message.source !== undefined && message.source.length < 1000;
+          var hasSource = message.source && message.source.length < 1000;
           if (hasSource) {
             for (var i = 0; i < message.column; i++) {
               if (message.source.charAt(i) === '\t') {

test/fixtures/fatal/results.json

@@ -5,6 +5,7 @@
       {
         "fatal": true,
         "severity": 2,
+        "source": null,
         "message": "Unexpected token {",
         "line": 3,
         "column": 13