Fix over-sized allocation of rtree leaf nodes.

Use the correct level metadata when allocating child nodes so that leaf
nodes don't end up over-sized (2^16 elements vs 2^4 elements).

src/rtree.c

@@ -128,5 +128,5 @@ rtree_node_elm_t *
 rtree_child_read_hard(rtree_t *rtree, rtree_node_elm_t *elm, unsigned level)
 {
 
-	return (rtree_node_init(rtree, level, &elm->child));
+	return (rtree_node_init(rtree, level+1, &elm->child));
 }