Check the prunelink member of the correct node

Check the prunelink member of the correct node

Commit 4b6fc97af6f936616a12e733b14ffc450af6df87 checks the prunelink
member of the node that was just pruned, not its parent node that was
intended to be examined.  Fix by checking the prunelink member of the
parent node, so that adding the latter to its relevant prunenodes list
twice is properly guarded against.