Add a NULL check to protect the situation below(may only happened in the case of unit test):
1.A new datanode register to SCM.
2. There is no container allocated in the new datanode temporarily.
3.The new datanode dead and an event was fired to DeadNodeHandler
4.In function DeadNodeHandler#onMessage, there will get nothing in node2ContainerMap and containers will be NULL
5.NullPointerException will be throwen in the following iterate of containers like: