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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/*

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Template C code used to produce target Ethercat C code.

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Copyright (C) 2011-2014: Laurent BESSARD, Edouard TISSERANT

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Distributed under the terms of the GNU Lesser General Public License as

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published by the Free Software Foundation; either version 2 of the License, or

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(at your option) any later version.

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See COPYING file for copyrights details.

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*/

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#include "iec_types_all.h"

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#define FREE 0

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#define ACQUIRED 1

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#define ANSWERED 2

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long SDOLock = FREE;

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extern long AtomicCompareExchange(long* atomicvar,long compared, long exchange);

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int AcquireSDOLock() {

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	return AtomicCompareExchange(&SDOLock, FREE, ACQUIRED) == FREE;

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}

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void SDOAnswered() {

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	AtomicCompareExchange(&SDOLock, ACQUIRED, ANSWERED);

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}

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int HasAnswer() {

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	return SDOLock == ANSWERED;

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}

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void ReleaseSDOLock() {

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	AtomicCompareExchange(&SDOLock, ANSWERED, FREE);

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}

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int __init_etherlab_ext()

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{

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    SDOLock = FREE;

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    return 0;

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}

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void __cleanup_etherlab_ext()

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{

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}

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void __retrieve_etherlab_ext()

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{

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}

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void __publish_etherlab_ext()

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{

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}