[FIX] website_sale: potential crash

`_cart_find_product_line` can return more than 1 order line . Since it
might be necessary in `website_sale_options`, just prevent the crash.

opw-743805

addons/website_sale/models/sale_order.py

@@ -71,7 +71,7 @@ class SaleOrder(models.Model):
         if order.pricelist_id and order.partner_id:
             order_line = order._cart_find_product_line(product.id)
             if order_line:
-                pu = self.env['account.tax']._fix_tax_included_price(pu, product.taxes_id, order_line.tax_id)
+                pu = self.env['account.tax']._fix_tax_included_price(pu, product.taxes_id, order_line[0].tax_id)
 
         return {
             'product_id': product_id,