svn merge -c 1525828 from trunk for HDFS-5228. The RemoteIterator returned by DistributedFileSystem.listFiles may throw NullPointerException.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.1.1-beta@1525839 13f79535-47bb-0310-9956-ffa450edef68