[BUG] Stock of BE variants is not considered when adding product to cart

[rintisch]

This is about cart_products version 5.x-dev (as linked below)

How to recreate the bug:

• Activate stock handling of a product
• In BE: Set quantity of product to 1 and save the product.
• In FE: Refresh the detail page with the product → The product is available and can be added to the cart (but do not do it yet!)
• In BE: Update the quantity of the product to 0 and save the product again.
• In FE: Put the product into the cart.
• Error occured: The product could be placed in the cart.

This can happen is two customer add the last product at nearly the same time.