Menu: don't set focusedItem to values lower than -1

Menu: don't set focusedItem to values lower than -1

It fixes this subtle bug: QC crash by "dragging" any item in Settings > Game's ListBox (the thing with HUD,
Messages, etc.) above the top of the list (even though it's an immutable list, so dragging should do
nothing other than change the selection).
It was due to XonoticRegisteredSettingsList_focusedItemChangeNotify considering focusedItems < -1
as valid items.
For extra safety I made so that XonoticRegisteredSettingsList_focusedItemChangeNotify considers any
focusedItem < 0 invalid anyway (like XonoticGametypeList_focusedItemChangeNotify).

I also removed a duplicated focusedItemChangeNotify declaration.