Duplicate callback invocation on error.

parser.addListener: end and error both trigger callback if there is trouble with xml.

``` javascript
parser.addListener('end', function(jsonDOM) {
 if (jsonDOM) {
 //...
 if (callback) callback(null, output);
 } else {
 if (callback) callback("failed to parse xml", null);
 }
}

parser.addListener("error", function(err) {
 // ...
 callback = null;
});
```
