fix cmdu_ackq_dequeue crash for NULL cookie

src/cmdu_ackq.c

@@ -317,7 +317,8 @@ int cmdu_ackq_dequeue(void *cmdu_q, uint16_t type, uint16_t mid, uint8_t *src, v
 
 
 	/* After returning cookie back to user, we can safely delete the msg */
-	*cookie = msg->cookie;
+	if (cookie)
+		*cookie = msg->cookie;
 	msg->cookie = NULL;
 
 	cmdu_ackq_delete_msg(q, msg);