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

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