[FIX] website_sale_stock(_option): prevent negative stock & traceback

Before this commit, multiple tabs/session adding items to cart  could lead to
negative stock displayed in the quantity field.
In some case, it could throw a traceback because the wanted product had
became unavailable in the meantime, causing the cart to be updated and the
line to be deleted from the cart. But then, this line was being accessed again.

Now, negative values are prevented at the screen. And there is a check on the
line record to be sure it exists before accessing it.