Builtin/../falling.lua: Avoid crash when object pos over limit

If the object pos is over limit, 'add entity' will not add an entity,
causing 'obj' to be nil.

builtin/game/falling.lua

@@ -113,7 +113,9 @@ core.register_entity(":__builtin:falling_node", {
 
 local function spawn_falling_node(p, node)
 	local obj = core.add_entity(p, "__builtin:falling_node")
+	if obj then
 		obj:get_luaentity():set_node(node)
+	end
 end
 
 local function drop_attached_node(p)