HADOOP-3509. Fix NPE in FSNamesystem.close. Contributed by Tsz Wo (Nicholas), SZE.

Files:
modified: /hadoop/core/trunk/src/java/org/apache/hadoop/dfs/FSDirectory.java (try)
modified: /hadoop/core/trunk/src/java/org/apache/hadoop/dfs/FSNamesystem.java (try)
modified: /hadoop/core/trunk/CHANGES.txt (try)
modified: /hadoop/core/trunk/src/java/org/apache/hadoop/io/IOUtils.java (try)