fix: crash when a script menu update renders selected index unavailable

[po5]

When a script-defined menu gets updated with fewer items than it previously had, the user's selected index doesn't get clamped or reset, leading to this crash when pressing Enter:

[uosc] 
[uosc] stack traceback:
[uosc]  /home/eva/.config/mpv/scripts/uosc/elements/Menu.lua:499: in function 'open_selected_item'
[uosc]  /home/eva/.config/mpv/scripts/uosc/elements/Menu.lua:514: in function </home/eva/.config/mpv/scripts/uosc/elements/Menu.lua:514>
[uosc]  (...tail calls...)
[uosc]  /home/eva/.config/mpv/scripts/uosc/elements/Menu.lua:678: in function 'fn'
[uosc]  mp.defaults:230: in function 'fn'
[uosc]  mp.defaults:65: in function 'handler'
[uosc]  mp.defaults:380: in function 'handler'
[uosc]  mp.defaults:510: in function 'call_event_handlers'
[uosc]  mp.defaults:552: in function 'dispatch_events'
[uosc]  mp.defaults:503: in function <mp.defaults:502>
[uosc]  [C]: in ?
[uosc]  [C]: in ?
[uosc] Lua error: /home/eva/.config/mpv/scripts/uosc/elements/Menu.lua:499: attempt to index local 'item' (a nil value)

I'm not sure if Menu:update_dimensions() is the proper place to update the selected index, but it fixes the issue.
Ran into this with memo, switching from the 2nd page of results back to the first, if you need a real example of it.

[christoph-heinrich]

Menu:update() is a more appropriate place for that imo.

[po5]

I tried that before and it doesn't fix the crash.

[po5]

Done.

[tomasklaen]

Do a:

menu.selected_index = #menu.items > 0 and clamp(1, menu.selected_index, #menu.items) or nil

[po5]

(:

[tomasklaen]

No no, the condition was fine, since this shouldn't run when menu.selected_index is nil. I don't think clamp() handles nils.

[po5]

Should be good now.