AFNetworking/AFNetworking — pull request #294 by kirsplatrick (2012-04-11 23:57:43+00:00)
Title: Fixed crash in special case for custom callback queues

This update is to fix a fairly obscure crash that occurs when using custom callback queues. The code below will currently crash the application when the completion block is called with a success. The same will occur with the failure block if the failureCallbackQueue is set the same way as below.

As a side note, If `queue` is not released then the app will not crash, but the completion block will be called on the queue that was originally set.

```
// Create AFHTTPRequestOperation called "operation"

dispatch_queue_t queue = dispatch_queue_create("aQueue", DISPATCH_QUEUE_CONCURRENT);

[operation setSuccessCallbackQueue:queue];
[operation setSuccessCallbackQueue:NULL];

dispatch_release(queue);

// Fire operation
```