HDFS-4482. ReplicationMonitor thread can exit with NPE due to the race between delete and replication of same file. Contributed by Uma Maheswara Rao G.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-0.23@1518834 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-0.23' into branch-0.23-gd
Merge remote branch 'origin/branch-0.23-gd' into fb-HADOOP-9166-branch-0.23-gd