Merge r1587778 from branch-2. 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-2.4@1587779 13f79535-47bb-0310-9956-ffa450edef68