DERBY-6348: NPE or assert failure in recursive trigger

Reset the activation in the statement context before setting up the
activation for the next trigger, so that the nested trigger activation
is a child of the top-level activation, and not a child of the
previous trigger's activation.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1537393 13f79535-47bb-0310-9956-ffa450edef68