[FIX] account: traceback on onchange of company in invoice.

I have fixed the traceback : (with multicompany group on user) Go to -> Accounting -> Customer/Supplier invoice, create new invoice do not select customer/supplier and change company it will throws traceback.

- Task : https://www.odoo.com/web#id=9168&view_type=form&model=project.task&menu_id=3942&action=327

- Pad : https://pad.odoo.com/p/openerp-project.task-BDDX2XL85U