[9.16] 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.

Closes #4596 (closed)