[FIX] purchase: confirm only draft/sent PO

Without this fix, it is possible to confirm a PO which is not in state
'draft' and 'sent'. The issue is that it will create a picking at every
call.

The issue was occurring in Purchase Requisition, which was fixed in
previous commit. However, it makes sense to add a check directly in the
`button_confirm` method.

opw-684347

addons/purchase/purchase.py

@@ -281,6 +281,8 @@ class PurchaseOrder(models.Model):
     @api.multi
     def button_confirm(self):
         for order in self:
+            if order.state not in ['draft', 'sent']:
+                continue
             order._add_supplier_to_product()
             # Deal with double validation process
             if order.company_id.po_double_validation == 'one_step'\