update_icon_pixmap don't paint dock icon

The dock applications has their icon image set in icon->file_image
to draw the icon if the application is not running. But if the
application is running, the icon mustn't be show. This patch don't
paint the icon for docks.

src/icon.c

@@ -640,7 +640,7 @@ void update_icon_pixmap(WIcon *icon)
 	/* If dockapp, put inside the icon */
 	if (icon->icon_win != None) {
 		/* file_image is NULL, because is docked app */
-		icon_update_pixmap(icon, icon->file_image);
+		icon_update_pixmap(icon, NULL);
 		set_dockapp_in_icon(icon);
 	}