DERBY-6693 Assert failure/ArrayIndexOutOfBoundsException when using COUNT in MERGE matching clause

Backported from trunk cleanly as:

svn merge -c 1616523 https://svn.apache.org/repos/asf/db/derby/code/trunk

The patch forbids aggregates in the matching clauses, and adds a test
case to MergeStatementTest.



git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/branches/10.11@1616529 13f79535-47bb-0310-9956-ffa450edef68