HDFS-5077. NPE in FSNamesystem.commitBlockSynchronization(). Contributed by Plamen Jeliazkov.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1518851 13f79535-47bb-0310-9956-ffa450edef68
Merge r1518851 from trunk (reconcile branch conflicts with the fix for HDFS-5077)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-2832@1519882 13f79535-47bb-0310-9956-ffa450edef68