[Product] Fixed traceback on calculation of cost price.

Hello,

I have fixed the traceback : Create new product and its bom(assign variant in bom) now, go to variant and try to compute cost price it gives traceback.

More details:

Task : http://www2.odoo.com/web#id=8652&view_type=form&model=project.task&menu_id=3942&action=327

Pad : https://pad.odoo.com/p/openerp-project.task-QC8RQH8QCA

Thanks,
cod-odoo