SVGHMI: Fixed typo on session manager unregister, leading to wrong count of sessions and then exceptions when creating more session than allowed in protocol options. Also added more safety check in protocol in case session would be missing.

#!/usr/bin/env python
# -*- coding: utf-8 -*-

# This file is part of Beremiz
# Copyright (C) 2019: Edouard TISSERANT
#
# See COPYING file for copyrights details.

from __future__ import absolute_import
from svghmi.svghmi import *