fix crash on get_instance_axis_value condition when...
fix crash on get_instance_axis_value condition when...

... a font lacks fvar table.
(issue #1723)

googlefonts/fontbakery,graphicore/fontbakery,graphicore/fontbakery,moyogo/fontbakery,graphicore/fontbakery,moyogo/fontbakery,googlefonts/fontbakery,moyogo/fontbakery,googlefonts/fontbakery