PIG-3786: POReservoirSample should handle endOfAllInput flag (daijy)


git-svn-id: https://svn.apache.org/repos/asf/pig/branches/tez@1574986 13f79535-47bb-0310-9956-ffa450edef68
PIG-3788: NPE when POStream is not in the leaf vertex (daijy)


git-svn-id: https://svn.apache.org/repos/asf/pig/branches/tez@1574991 13f79535-47bb-0310-9956-ffa450edef68