[syslinux] menu system behaviour with empty config file

[Sebastian Herbszt]

The behaviour of the menu system is odd when an empty / not existing config file is specified:
the screen is cleared but not even the menu frame is displayed.

It seems to be a problem with run_menu() from menumain.c:

        if (top != prev_top) {
            draw_menu(entry, top, 1);
            display_help(me->helptext);
        } else if (entry != prev_entry) {
            draw_row(prev_entry - top + 4 + VSHIFT, entry, top, 0, 0);
            draw_row(entry - top + 4 + VSHIFT, entry, top, 0, 0);
            display_help(me->helptext);
        }

And we have entry = prev_entry = top = prev_top = -1.

- Sebastian