[FIX] stock_account: partial reception with avg cost

- Create a product where the associated product category has Costing
  Method "Average Cost (AVCO)"
- Create a PO. Order 1 unit at $1000.
- Receive that unit. (System now has 1 unit at $1000).
- Create another PO. Order 3 units at $500 each.
- Receive only 1 unit and don’t create any back order.

System has 2 units @ $625, instead of $750.

The quantity computed should take into account the quantity actually
done.

opw-1843998