fix a bug in multi-LDAP authentication

see: https://github.com/ansible/tower/issues/2738

awx/sso/backends.py

@@ -110,6 +110,7 @@ class LDAPBackend(BaseLDAPBackend):
             pass
         try:
             user = super(LDAPBackend, self).authenticate(username, password)
+            if user and getattr(user, 'ldap_user', None):
                 try:
                     user.ldap_user._get_groups().get_group_dns()
                 except ImproperlyConfigured: