DERBY-4403: Disallow subqueries in the GROUP BY list

Queries that were grouped by subqueries used to fail with a
NullPointerException or an assert failure. Make them fail with a
proper syntax error instead.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1575226 13f79535-47bb-0310-9956-ffa450edef68