itree.c: Clarify how the sentinel is used

itree.c: Clarify how the sentinel is used

* src/itree.c (null_is_sane): New function.
(interval_tree_iter_start): Use it to make sure we haven't garbled null.
(itree_init): Fully initialize `itree_null` here...
(interval_tree_clear): ...instead of here.
(interval_tree_propagate_limit): Remove special code that read NULL->parent.
(interval_tree_remove): Do it explicitly before the call in those two
places where it can happen.