Fix access to undefined response

Previous code caused TypeError: Cannot read property 'pap' of undefined in case of errors.

See https://github.com/Leonidas-from-XIV/node-xml2js/blob/master/lib/xml2js.js#L400 for reference.

Here is the excerpt from our log which lead me to looking into this:
```
/opt/node_modules/node-bb10/lib/PushInitiator.js:116
                        if(res.pap !== undefined) {
                              ^
TypeError: Cannot read property 'pap' of undefined
    at /opt/node_modules/node-bb10/lib/PushInitiator.js:116:17
    at Parser.<anonymous> (/opt/node_modules/node-bb10/node_modules/xml2js/lib/xml2js.js:265:20)
    at Parser.emit (events.js:95:17)
```