Apply bug fix from upstream clipper

Clipper fix in commit [r463] for bug repport #92 (http://sourceforge.net/p/polyclipping/code/463/tree//trunk/cpp/clipper.cpp?diff=504b9404fd48f873331e913b:462)
is applied here.

This fix should be quite safe to apply, it does only modify part of the code causing bug, in most cases it is not executed at all.