HIVE-6618 : assertion when getting reference key from loader with byte-array mapjoin key (Sergey Shelukhin, reviewed by Gunther Hagleitner)

git-svn-id: https://svn.apache.org/repos/asf/hive/branches/branch-0.13@1577274 13f79535-47bb-0310-9956-ffa450edef68