The implementation of RBTree_Next was using an awkward construction to detect
and avoid accessing the false root of the red-black tree. To deal with the
false root, RBTree_Next was comparing node parents with the control node.
Instead the false root can be detected by checking if the grandparent of a
node exists; the grandparent of the tree's true root is NULL by definition
so the root of the tree is found while walking up the tree by checking for
the non-existence of a grandparent.
This change propagates into the predecessor/successor and iterate functions.
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