YARN-1128. FifoPolicy.computeShares throws NPE on empty list of Schedulables (Karthik Kambatla via Sandy Ryza)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1525313 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-HDFS-4311-trunk-gd