fix bug at the time of the payment when stock=0 and checkAvailableStock=0

if stock=0 and checkAvailableStock=0
and forbid negative stock
But for future management
> Stock notifications
>
>    Alert if the stock level is low. Alerts are sent by mail. Threshold must be defined for each product or product attribute.

It would not be better to continue to update the stock ?

if not I modify the script