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

if stock=0 and checkAvailableStock=0
and forbid negative stock
I don't undestand
> But there's a condition:
> a ∈ [1, +∞],
> so a > 0 is always false and throws an exception.

a > 0 is always true
We cannot order a product with a quantity equal to 0       right ?

a -> cartitem = 10
checkAvailableStock = 0
b -> stock = 0

if b - a < 0       it's possible, right ?