HDFS-4867. metaSave NPEs when there are invalid blocks in repl queue. Contributed by Plamen Jeliazkov and Ravi Prakash.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-0.23@1490436 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-0.23' into branch-0.23-gd