Gets the upper bound node corresponding to the given key,
or %NULL if the tree is empty or all the nodes in the tree
have keys that are lower than or equal to the searched key.
The upper bound is the first node that has its key strictly greater
than the searched key.
Gets the upper bound node corresponding to the given key, or %NULL if the tree is empty or all the nodes in the tree have keys that are lower than or equal to the searched key.
The upper bound is the first node that has its key strictly greater than the searched key.