[FIX] website: prevent /web/become route to crash

Since 9f82605df1, any call to `_login_redirect()` without previously going
through `web_login()` method would crash, as `login_success` is set there.

In this case, it was:
Dispatch -> web_login -> redirect (new dispatch) -> _login_redirect()

task-2340941

closes odoo/odoo#57881

X-original-commit: 448fcb3c702c3346ca5af922bbf457231a7330f5
Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>

addons/website/controllers/main.py

@@ -99,7 +99,7 @@ class Website(Home):
         """ Redirect regular users (employees) to the backend) and others to
         the frontend
         """
-        if not redirect and request.params['login_success']:
+        if not redirect and request.params.get('login_success'):
             if request.env['res.users'].browse(uid).has_group('base.group_user'):
                 redirect = b'/web?' + request.httprequest.query_string
             else: