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'

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#!/usr/bin/env python

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# -*- coding: utf-8 -*-

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# See COPYING file for copyrights details.

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from __future__ import absolute_import

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import hashlib

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class ConnectorBase(object):

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    chuncksize = 1024*1024

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    def BlobFromFile(self, filepath, seed):

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        s = hashlib.new('md5')

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        s.update(seed)

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        blobID = self.SeedBlob(seed)

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        with open(filepath, "rb") as f:

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            while blobID == s.digest():

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                chunk = f.read(self.chuncksize)

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                if len(chunk) == 0:

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                    return blobID

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                blobID = self.AppendChunkToBlob(chunk, blobID)

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                s.update(chunk)