title: app crashes because of null values, swift 2
body: <p>I'm trying to get data using JSON ad there is a null value causes crash.</p>
  
  <pre><code>let jsonData:NSDictionary = try NSJSONSerialization.JSONObjectWithData(urlData!, options:NSJSONReadingOptions.MutableContainers ) as! NSDictionary
  
  let success:NSInteger = jsonData.valueForKey("success") as! NSInteger
  //code error bad instruction
  
  NSLog("Success: %ld", success);
  </code></pre>
  
  <p>how can I turn null to zero?</p>