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

These classes are intended for use server-side.

e.g. in a django view.py :

    from pyjs.jsonrpc.django import JSONService, jsonremote

    jsonservice = JSONRPCService()

    @jsonremote(jsonservice)
    def test(request, echo_param):
         return "echoing the param back: %s" % echo_param