[AXE-1443] Timing issue is resolved: _tearDown is called from _attemptToReconnect -> delegate is notified about an error via calling amqpConsumerThread:reportedError: asynchronously so the delegate cancel the thread -> in the meantime one more iteration of _consume method happens -> amqp_maybe_release_buffers is called for NULL object.
Now .AMQPConsumerThread is being cancelled while handling an internal error (it doesn't rely only on its delegate anymore).