Fix segmentation fault due to 'write_vtk'.
Fix segmentation fault due to 'write_vtk'.

This segmentation fault arose due to the first call to 'write_vtk' in the example script 'doc/examples/viennagrid_wrapper/accessors.py'.

jonancm/viennagrid-python,jonancm/viennagrid-python,jonancm/viennagrid-python