DERBY-6674 Cleanup brittle code in ValidateCheckConstraintResultSet

Patch derby-6674. Refactors the code by reusing getNextRowCore (to
avoid the redundancy) for ValidateCheckConstraintResultSet but
abstracting out the loop control, which is overridden by
ValidateCheckConstraintResultSet.

fix

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1614963 13f79535-47bb-0310-9956-ffa450edef68