Fix: Use newgame ending year on highscore table when not in a game.

[PeterN]

Motivation / Problem

Highscore table uses the game end year from the _settings_game settings even when in the intro screen. This (probably) displays the intro game's end year instead of the configured end year.

Description

When not in a game, use the _settings_newgame end year setting instead.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[PeterN]

This change is pointless if #11546 is preferred.