Fix an assert in isCopperOutside.

pcbnew/convert_shape_list_to_polygon.cpp

@@ -159,6 +159,8 @@ static bool isCopperOutside( const FOOTPRINT* aFootprint, SHAPE_POLY_SET& aShape
     {
         SHAPE_POLY_SET poly = aShape.CloneDropTriangulation();
 
+        poly.ClearArcs();
+
         poly.BooleanIntersection( *pad->GetEffectivePolygon( ERROR_INSIDE ),
                                   SHAPE_POLY_SET::PM_FAST );