Guard clause: redirect to /guest when POST

When there is no connected user, and when the distributor doesn't allow guest orders, redirect on POST method

app/controllers/split_checkout_controller.rb

@@ -22,6 +22,8 @@ class SplitCheckoutController < ::BaseController
   end
 
   def update
+    return redirect_to_guest if !spree_current_user && !@order.distributor.allow_guest_orders?
+
     if confirm_order || update_order
       clear_invalid_payments
       advance_order_state