[FIX] stock: prevent location change

Changing the partner on a picking triggers an onchange which might
modify `location_id` or `location_dest_id`. However, when the picking is
not draft anymore, these fields are not supposed to be changed.

opw-1872635

addons/stock/models/stock_picking.py

@@ -470,8 +470,9 @@ class Picking(models.Model):
             else:
                 location_dest_id, supplierloc = self.env['stock.warehouse']._get_partner_locations()
 
-            self.location_id = location_id
-            self.location_dest_id = location_dest_id
+            if self.state == 'draft':
+                self.location_id = location_id
+                self.location_dest_id = location_dest_id
         # TDE CLEANME move into onchange_partner_id
         if self.partner_id:
             if self.partner_id.picking_warn == 'no-message' and self.partner_id.parent_id: