Warn about shovel only when one exists

queue.c

@@ -61,7 +61,12 @@ static void queue_destructor(node_t *n) {
       }
     }
     destroy_hashtable(&q->subscriptions);
-    warn("TODO: the shovel needs to be taken down as well here\n");
+    if (q->shovel) {
+      warn("TODO: the shovel needs to be taken down as well here\n");
+      /* The difficulty is that the shovel may be running at the
+	 moment, so careful ordering of operations is required to
+	 avoid referencing deallocated memory. */
+    }
     free(q);
   }
 }