[smooth] do not collect incident cells for non free vertices

Tetrahedral_remeshing/include/CGAL/Tetrahedral_remeshing/internal/smooth_vertices.h

@@ -198,6 +198,7 @@ private:
       for (auto vi : tr.vertices(c))
       {
         const std::size_t idi = vertex_id(vi);
+        if(is_free(idi))
           inc_cells[idi].push_back(c);
       }
     }