clipper: Add a constructor for PolyNode.

Missing constructor for the PolyNode class leads to uninitialised
pointer use in some cases.

Filed upstream as http://sourceforge.net/p/polyclipping/bugs/64/
geometry: Silence a compiler warning due to dangling code.

Code below the final return statement in fit_arc() was leading to
a compiler warning about reaching the end of a non-void function.
Comment it out.