[FIX] stock_account: Changing costing method with negative stock

Usecase to reproduce:
- Set product as FIFO and a cost of 10$
- Sell 5 units
- Modify the cost method to average

We expect the value of the stock to remain -50$.
However it went to -150$

It happens because the change of cost method is not supported
with negative quantity of stock. It's not really a huge bug because
it will be reconcile to the real value in the next in and when the
stock will be positive again but at least it gives a proper valuation
of the inventory.

opw-3100717

closes odoo/odoo#111341

X-original-commit: 3ff51daa
Signed-off-by: Arnold Moyaux (arm) <arm@odoo.com>