itree.c: Get rid of the trick using null->parent

itree.c: Get rid of the trick using null->parent

* src/itree.c (interval_tree_remove_fix): Add a `parent` argument.
Change the loop so it always keeps both `node` and `parent` in sync,
thus avoiding the use of `node->parent` on the initial node (since
that one can be null).
(interval_tree_remove): Manually keep track of the `broken` node's
parent to pass it to `interval_tree_remove_fix`.