show backends ERROR 1064 (HY000):

[imay]

Describe the bug
When a backend is down, execute show backends will get ERROR 1064 (HY000):

To Reproduce
Steps to reproduce the behavior:

1. kill a backend
2. execute show backends in mysql client
3. get ERROR 1064 (HY000):

Expected behavior
Return all backends status include the crush backend.

Useful Log

2018-11-15 18:43:21,822 INFO 384 [BackendsProcDir.getClusterBackendInfos():176] backends proc get tablet num cost: 0, total cost: 0
2018-11-15 18:43:21,822 WARN 384 [StmtExecutor.execute():284] execute Exception
java.lang.NullPointerException
        at org.apache.doris.common.proc.BackendsProcDir.getClusterBackendInfos(BackendsProcDir.java:186) ~[palo-fe.jar:?]
        at org.apache.doris.qe.ShowExecutor.handleShowBackends(ShowExecutor.java:973) ~[palo-fe.jar:?]
        at org.apache.doris.qe.ShowExecutor.execute(ShowExecutor.java:186) ~[palo-fe.jar:?]
        at org.apache.doris.qe.StmtExecutor.handleShow(StmtExecutor.java:723) ~[palo-fe.jar:?]
        at org.apache.doris.qe.StmtExecutor.execute(StmtExecutor.java:262) [palo-fe.jar:?]
        at org.apache.doris.qe.ConnectProcessor.handleQuery(ConnectProcessor.java:161) [palo-fe.jar:?]
        at org.apache.doris.qe.ConnectProcessor.dispatch(ConnectProcessor.java:251) [palo-fe.jar:?]
        at org.apache.doris.qe.ConnectProcessor.processOnce(ConnectProcessor.java:382) [palo-fe.jar:?]
        at org.apache.doris.qe.ConnectProcessor.loop(ConnectProcessor.java:392) [palo-fe.jar:?]
        at org.apache.doris.qe.ConnectScheduler$LoopHandler.run(ConnectScheduler.java:172) [palo-fe.jar:?]
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_131]
        at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_131]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [?:1.8.0_131]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [?:1.8.0_131]
        at java.lang.Thread.run(Thread.java:748) [?:1.8.0_131]