wacom: Allow button mapping if the stylus has one button too

The airbrush tool has only one button, and the button mapping combo box
does not work.
We should not bail out if the widget has been removed, that means that
the stylus does not support the second button.

Check upfront the number of buttons to know if the top button parameter
is available.

https://bugzilla.gnome.org/show_bug.cgi?id=746263

panels/wacom/cc-wacom-stylus-page.c

@@ -202,12 +202,16 @@ button_changed_cb (GtkComboBox *combo, gpointer user_data)
 			    BUTTONNUMBER_COLUMN, &mapping_b2,
 			    -1);
 
-	if (!gtk_combo_box_get_active_iter (GTK_COMBO_BOX (WID ("combo-topbutton")), &iter))
-		return;
+	if (gsd_wacom_stylus_get_num_buttons (priv->stylus) > 1) {
+		if (!gtk_combo_box_get_active_iter (GTK_COMBO_BOX (WID ("combo-topbutton")), &iter))
+			return;
 
-	gtk_tree_model_get (GTK_TREE_MODEL (liststore), &iter,
-			    BUTTONNUMBER_COLUMN, &mapping_b3,
-			    -1);
+		gtk_tree_model_get (GTK_TREE_MODEL (liststore), &iter,
+				    BUTTONNUMBER_COLUMN, &mapping_b3,
+				    -1);
+	} else {
+		mapping_b3 = 0;
+	}
 
 	map_button (priv->stylus_settings, mapping_b2, mapping_b3);
 }