Do not re-add a node to the same prunenodes list

Do not re-add a node to the same prunenodes list

If a node cleaned up by prune_tree() happens to belong to the same node
bucket as its parent, the latter is directly appended to the prunenodes
list currently processed by prune_tree().  However, the relevant code
branch does not account for the fact that the parent might already be on
the list it is trying to append it to.  Fix by only calling
ISC_LIST_APPEND() for parent nodes not yet added to their relevant
prunenodes list.