Celery.close should not create amqp object if not already created

celery/app/base.py

@@ -446,8 +446,8 @@ class Celery(object):
         if self._pool:
             self._pool.force_close_all()
             self._pool = None
-            amqp = self.amqp
-            if amqp._producer_pool:
+            amqp = self.__dict__.get('amqp')
+            if amqp is not None and amqp._producer_pool is not None:
                 amqp._producer_pool.force_close_all()
                 amqp._producer_pool = None