(itree_insert_gap, itree_delete_gap): Minor optimization

(itree_insert_gap, itree_delete_gap): Minor optimization

`limit` can get smaller in either of the two children of a node.
It can also happen that the root node itself has a low enough limit
that the loop can be interrupted right away.

The previous code only checked `limit` when going down to a left
child, which is not wrong, but tests suggest that it is also very
common to reach this limit when going to a right child, so move the
test accordingly.

* src/itree.c (itree_insert_gap, itree_delete_gap): Check `limit` for
all nodes, rather than only when following a `left` pointer.