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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#ifndef _BEREMIZ_H_

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#define _BEREMIZ_H_

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/* Beremiz' header file for use by extensions */

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#include "iec_types.h"

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#define LOG_LEVELS 4

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#define LOG_CRITICAL 0

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#define LOG_WARNING 1

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#define LOG_INFO 2

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#define LOG_DEBUG 3

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extern unsigned long long common_ticktime__;

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#ifdef TARGET_LOGGING_DISABLE

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static inline int LogMessage(uint8_t level, char* buf, uint32_t size)

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{

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	(void)level;

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	(void)buf;

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	(void)size;

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	return 0;

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}

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#else

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int     LogMessage(uint8_t level, char* buf, uint32_t size);

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#endif

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

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#endif