[Account] Fix raceback on onchange of company in invoice.

Hello,

I have fixed the traceback : Go to -> Accounting -> Customer/Supplier invoice, create new invoice do not select customer/supplier and change company it will throws traceback.

More details:

Task : http://www2.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

Thanks,
cod-odoo