[FIX] purchase: confirm of purchase orders with kit boms

When using phantom/kit boms,
`action_confirm` on the move can lead to the move unlink,
as new moves with the different parts of the product have
been created.

See `_action_explode` in `mrp/stock.py`

The method `action_confirm` on `stock.move` returns the list
of remaining moves.

`force_assign` should therefore be performed on this new
moves list, returned by `action_confirm`, rather than
be performed on the moves list sent to `action_confirm`

opw-660722

addons/purchase/purchase.py

@@ -337,7 +337,8 @@ class PurchaseOrder(models.Model):
                 res = order._prepare_picking()
                 picking = self.env['stock.picking'].create(res)
                 moves = order.order_line.filtered(lambda r: r.product_id.type in ['product', 'consu'])._create_stock_moves(picking)
-                moves.action_confirm()
+                move_ids = moves.action_confirm()
+                moves = self.env['stock.move'].browse(move_ids)
                 moves.force_assign()
         return True