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

          <xsd:attribute name="Command" type="xsd:string" use="optional" default="make -C %(buildpath)s all BEREMIZSRC=%(src)s BEREMIZCFLAGS=%(cflags)s MD5=%(md5)s USE_BEREMIZ=1 FROM_BEREMIZ=1"/>