middleware: improve condition to automatically determine a common domain (fixes #15548)

It works if:
- HTTP Host is a domain name and not an IP address (IPv6 address will not pass
  this test, they lack dots),
- domain contains at least three components.

mellon/middleware.py

@@ -35,8 +35,12 @@ class PassiveAuthenticationMiddleware(object):
             # get the common domain or guess
             common_domain = app_settings.OPENED_SESSION_COOKIE_DOMAIN
             if not common_domain:
+                host = request.get_host()
+                # accept automatic common domain selection if domain has at least three components
+                # and is not an IP address
+                if not host.count('.') > 1 or host.replace('.', '').isdigit():
+                    return
                 common_domain = request.get_host().split('.', 1)[1]
-                assert '.' in common_domain  # if domain is xxx.com explode !
             params = {
                 'next': request.build_absolute_uri(),
                 'passive': '',