[gsubgpos] Initialize variation index on HB_NO_VAR hb_shape_plan_key_t::equal expects hb_ot_shape_plan_key_t be initialized by hb_ot_layout_table_find_feature_variations calls but it won't get initialized when HB_NO_VAR build config is used. Related to https://github.com/harfbuzz/harfbuzz/issues/2280