rate_banner.cpp: try to fix possible crash if pixbuf = NULL

RestoreScreen() expects no null pointer passed as argument 5,
is passed to memmove

src/gui/rate_banner.cpp

@@ -80,6 +80,7 @@ void CRateBanner::paint(const bool &do_save_bg)
 	addIcons(rat_icon_bg, rat_max);
 
 	paintCCItems();
+	if (pixbuf)
 		frameBuffer->RestoreScreen(x_base, y, w_tmp, height, pixbuf);
 	delete[] pixbuf;
 	pixbuf = NULL;