[FIX] mass_mailing: crash with Firefox with the breadcrumb

- When opening a mass mailing
 - Editing it and making a change (e.g. add a block)
 - Click on the breadcrumb
 - Click "OK" on the warning popup telling your changes will be discarded
 - Choose another mass mailing

A crash occured. This is basically the same case than in the revision
7bf80d22dd
but for Firefox, for which `window` is still there
`!window` returns False
but is `closed`

opw-702211

addons/mass_mailing/static/src/js/mass_mailing_editor.js

@@ -235,7 +235,7 @@ snippets_editor.Class.include({
 var _set_value = window.top.odoo[callback+"_updown"];
 var odoo_top = window.top.odoo;
 window.top.odoo[callback+"_updown"] = function (value, fields_values, field_name) {
-    if (!window) {
+    if (!window || window.closed) {
         delete odoo_top[callback+"_updown"];
         return;
     }