DERBY-6565 ROW_NUMBER function throws NullPointerException in UPDATE statement

Patch *derby-6565-forbid.diff* which makes it illegal to use window
functions in the update source. Adds a new test for this error
condition. Also cleans up some Javadocs and modernizes some loops.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1615982 13f79535-47bb-0310-9956-ffa450edef68