paths2openscad Fedora 18 patch

This is just a really minor patch which makes thing #25036 (a very nice Inkscape extension created some moths ago by Dan Newman) work in Fedora 18 and probably all Inkscape versions > 0.48.4.

If not used, it just throws this exception:

terminate called after throwing an instance of 'std::logic_error'
what(): basic_string::_S_construct null not valid