Fix crash in runtime if PSK secret is missing

./Beremiz_service.py -s $PWD/psk2.txt -n beremiz /tmp/beremiz
Beremiz_service: 1.2-1378c18402c3+

Traceback (most recent call last):
File "./Beremiz_service.py", line 511, in <module>
ensurePSK(servicename, PSKpath)
File "/home/developer/WorkData/PLC/beremiz/beremiz/runtime/Stunnel.py", line 32, in ensurePSK
PSKgen(ID, PSKpath)
File "/home/developer/WorkData/PLC/beremiz/beremiz/runtime/Stunnel.py", line 23, in PSKgen
call(restart_stunnel_cmdline)
File "/home/developer/WorkData/PLC/beremiz/beremiz/runtime/spawn_subprocess.py", line 116, in call
pid = posix_spawn.posix_spawnp(cmd[0], cmd)
File "/home/developer/.local/lib/python2.7/site-packages/posix_spawn/_impl.py", line 120, in posix_spawnp
return _posix_spawn(lib.posix_spawnp, *args, **kwargs)
File "/home/developer/.local/lib/python2.7/site-packages/posix_spawn/_impl.py", line 111, in _posix_spawn
_check_error(res, path)
File "/home/developer/.local/lib/python2.7/site-packages/posix_spawn/_impl.py", line 10, in _check_error
raise OSError(errno, os.strerror(errno), path)
OSError: [Errno 2] No such file or directory: '/etc/init.d/S50stunnel'

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

# See COPYING file for copyrights details.

from __future__ import absolute_import

from itertools import repeat, islice, chain

from connectors.SchemeEditor import SchemeEditor

Schemes = ["WAMP", "WAMPS"]

model = [('host', _("Host:")),
         ('port', _("Port:")),
         ('realm', _("Realm:"))]


class WAMP_dialog(SchemeEditor):
    def __init__(self, *args, **kwargs):
        self.model = model
        self.EnableIDSelector = True
        SchemeEditor.__init__(self, *args, **kwargs)

    # pylint: disable=unused-variable
    def SetLoc(self, loc):
        hostport, realm, ID = list(islice(chain(loc.split("#"), repeat("")), 3))
        host, port = list(islice(chain(hostport.split(":"), repeat("")), 2))
        self.SetFields(locals())

    def GetLoc(self):
        fields = self.GetFields()

        # TODO : input validation test

        template = "{host}" + \
                   (":{port}" if fields['port'] else '') +\
                   "#{realm}#{ID}"

        return template.format(**fields)