Sketcher: [skip ci] fix segmentation fault in ViewProviderSketch::drawConstraintIcons()
Sketcher: [skip ci] fix segmentation fault in ViewProviderSketch::drawConstraintIcons()

For more details see: https://forum.freecadweb.org/viewtopic.php?f=19&t=54193

Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master