amplab/shark — pull request #297 by tianyi (2014-02-26 15:03:41+00:00)
Title: Null mergeCombiners will cause NPE in ExternalAppendOnlyMap

in most time , null mergeCombiners works fine if each key is different
from others, but if some key are duplicate , it will cause NPE when the
function mergeIfKeyExists is called in ExternalAppendOnlyMap.scala