Product with serial keys can still be bought if all the keys assigned to customers

[muhammadbtj]

Reproduction process:
Product having 2 keys and same customer will bought product 2 times, both keys assigned to that customer but if the same customer or any other customer buy that product he will not get any key and order is successfully placed.

in this case if product have keys and all the keys are assigned to customers then we have to restrict user to buy this product without having key.