font: Check return value from _cairo_ft_unscaled_font_lock_face

cairo-ft-font.c: In function ‘_cairo_ft_has_color_glyphs’:
cairo-ft-font.c:3011:9: warning: ignoring return value of ‘_cairo_ft_unscaled_font_lock_face’, declared with attribute warn_unused_result [-Wunused-result]
         _cairo_ft_unscaled_font_lock_face (unscaled);
         ^