Avoid division by zero in case width or height for page are not specified

traceback:
File "/home/developer/WorkData/PLC/beremiz/beremiz/editors/Viewer.py", line 3755, in OnPaint
self.DoDrawing(dc)
File "/home/developer/WorkData/PLC/beremiz/beremiz/editors/Viewer.py", line 3723, in DoDrawing
for y in xrange(self.PageSize[1] - (ystart * SCROLLBAR_UNIT) % self.PageSize[1], int(window_size[1] / self.ViewScale[1]), self.PageSize[1]):
<type 'exceptions.ZeroDivisionError'>: integer division or modulo by zero

/* 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
#define LogMessage(level, buf, size)
#else
int     LogMessage(uint8_t level, char* buf, uint32_t size);
#endif

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