Fix open polyline clipping

[hannukp]

Fixed to match original C# code (lines 1561-), fixes clipping of open polylines:

          foreach (OutRec outRec in m_PolyOuts)
          {
            if (outRec.Pts == null) 
                continue;
            else if (outRec.IsOpen)
                FixupOutPolyline(outRec);
            else
                FixupOutPolygon(outRec);
          }