проверка accessToken на nil

Поймал падение на 
[_preparedParameters setObject:token.accessToken forKey:VK_API_ACCESS_TOKEN];
![screenshot on 2014-05-07 at 13-29-49](https://cloud.githubusercontent.com/assets/340680/2900683/70c6c3b6-d5ca-11e3-9e59-1832c6c7ff97.png)