Merge r1587776 from trunk. YARN-1934. Fixed a potential NPE in ZKRMStateStore caused by handling Disconnected event from ZK. Contributed by Karthik Kambatla

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1587778 13f79535-47bb-0310-9956-ffa450edef68