NullPointerException occurs when query from subqueries with order by

Context may not have table scan which is created by OLAPJoinRel, so it does not have realization. In org.apache.kylin.query.relnode.OLAPSortRel#findMeasure, it visits "this.context.realization" directly, so makes the NullPointerException. For example, query like "select a.LSTG_FORMAT_NAME from (select LEAF_CATEG_ID,LSTG_FORMAT_NAME from KYLIN_SALES)a, (select LEAF_CATEG_ID,PART_DT from KYLIN_SALES)b order by a.LSTG_FORMAT_NAME".

Attached is the patch to fix, added a null protection.