[FIX] stock: error on stock.partial.move creation

The stock_partial_move wizard removes the required attribute for the field picking_id on a stock.partial.move. This means that we could get moves without picking_id and the previous line was failing ('NoneType' object has no attribute 'currency_id'). opw 614531
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-win32