check against undefined result in #requestAll

received error *TypeError: Cannot read property 'next_page' of undefined at `node-zendesk/lib/client/client.js:202:28`*

seems that `result` sometimes come back undefined and dies when anything is called on it. 
Some quick code tracing suggests that result  may be null due to a funky response.  `results` seems to come from `Client.prototype.request`, and it looks to me like it's created in the try/catch around line 124.  Result is null when there's an error parsing the JSON response.

To me this suggests that something is more fundamentally wrong with the library or the requests it generates or maybe even zendesk's response.  At the very least, it would be good to log some of this info so we can investigate more... this suggests that if `result` is null, it's not that we're not getting results, it's that something is wrong with the data.

@blairanderson have you been able to reproduce this case and see what might be causing it?  Or, can you add some logging that logs query info when this case is detected?