[FIX] purchase: skip move if not linked to a purchase order

When this situation happens. the 'cost' is None and the web interface cannot handle this value, provoking a JS error. Thus, prefer to fallback on the standard way to get the cost: based on the current standard price of the product.
Fixes #1032
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-win32