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

if stock=0 and checkAvailableStock=0
and forbid negative stock
Could you give me a case where this will be used please ?
There's already a comparison on line 225:
```php
$cartItem->getQuantity() > $pse->getQuantity()
```
let $cartItem->getQuantity() be "a" and $pse->getQuantity() be "b"
if a >= b, a - b >= 0, so a - b < 0 will never happen, right ?