range: don't draw origin when the slider is invisible

When the range of the GtkRange is zero (i.e. the upper and lower bounds
of the adjustment have the same value), don't use an origin to draw the
trough, as the slider will also be hidden, and the juncture between the
two sections of the trough will be visible.

gtk/gtkrange.c

@@ -2117,7 +2117,7 @@ gtk_range_draw (GtkWidget *widget,
 
       if (draw_trough)
         {
-          if (!priv->has_origin)
+          if (!priv->has_origin || !draw_slider)
             {
               gtk_render_background (context, cr,
                                      x, y, width, height);