[FIX]stock_landed_costs: Fixed wrong costs on landed costs with many pro...

Fixed wrong costs on landed costs with many products.
I've found an error because this was rewriting costs and this was creating wrong costs on quants.

Example:
For cycle 1: quant.id = 5
quant_dict = {5: 150}

for key, value in quant_dict.items():
    -> quant_obj.write(cr, uid, 5, {'cost': 150}, context=context)


For cycle 2: quant.id = 6
quant_dict = {6: 400, 5:150}

for key, value in quant_dict.items():
    -> quant_obj.write(cr, uid, 6, {'cost': 400}, context=context)
    -> quant_obj.write(cr, uid, 6, {'cost': 150}, context=context)


This doesn't happen all the time. But it's happening when quant_dict is all messed up, so all the costs are wrong.