Proper rightmost node fetch in zone tree.

This was wrong for cases like NSEC3 where no common suffix, like
the apex, is not in zone. Since the nodes are linked in circular
fashion, the easy solution is to find leftmost node and take it's
predecesor, which should be the rightmost node.

Change-Id: I5fa3714a889f159c682b255aeeef7d466f8be2ce