svn merge -c 1586034 from trunk for HDFS-6206. Fix NullPointerException in DFSUtil.substituteForWildcardAddress.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.4@1586038 13f79535-47bb-0310-9956-ffa450edef68