[Trunk]: Product: Supplier Information is generated randomly on the product form

How to Reproduce:
  - Customize product form view to show seller related info,
  - create multi supplier with same sequence,
  - refresh product form recursively, you can show a diffrent supplier info generate every time.

For more reference i attached video.
 I cheked the problem in _calc_seller function It is sorted partner list with sequence, if you remove this sorted function its working fine
 partner_list = sorted([(partner_id.sequence, partner_id)
                                       for partner_id in product.seller_ids
                                       if partner_id and isinstance(partner_id.sequence, (int, long))])

Thanks