Stock logic error when switching from unlimited to limited stock

[marlena-b]

There is a bug in the stock management system when a product initially set to unlimited stock is later assigned a limited stock quantity.

Reproduction steps:

1. Create a product with unlimited stock.
2. Create an order and add 5 units of the product, then pay for the order.
3. Change the product's stock (supply stock) to a value of 3.
4. Note that the product list now shows a stock of 3.
5. Create a new order and attempt to add the product.
6. Observe an error message indicating that the product is out of stock.