Merge r1499888 from branch-2 to branch-2.1-beta. YARN-845. RM crash with NPE on NODE_UPDATE (bikas)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.1-beta@1499889 13f79535-47bb-0310-9956-ffa450edef68