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;
});
```
