Prevent zero paper width or height

[Hartmut Holzgraefe]

Not sure how that was even possible, but somehow a paper size of zero by zero millimeters was submitted for eg:

http://print.get-map.org/maps/225033

leading to an "error writing to output stream" on rendering

[Hartmut Holzgraefe]

See also hholzgra/ocitysmap#55