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'

#ifndef _BEREMIZ_H_
#define _BEREMIZ_H_

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

#include "iec_types.h"

#define LOG_LEVELS 4
#define LOG_CRITICAL 0
#define LOG_WARNING 1
#define LOG_INFO 2
#define LOG_DEBUG 3

extern unsigned long long common_ticktime__;

#ifdef TARGET_LOGGING_DISABLE
static inline int LogMessage(uint8_t level, char* buf, uint32_t size)
{
	(void)level;
	(void)buf;
	(void)size;
	return 0;
}
#else
int     LogMessage(uint8_t level, char* buf, uint32_t size);
#endif

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

#endif