dbus_server_listen: Assert that implementations return a known result

If they didn't, we'd probably leak the server and/or the error.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=107194

dbus/dbus-server.c

@@ -631,6 +631,10 @@ dbus_server_listen (const char     *address,
               
               /* keep trying addresses */
             }
+          else
+            {
+              _dbus_assert_not_reached ("Unknown result in dbus_server_listen");
+            }
         }
 
       _dbus_assert (server == NULL);