Fix #9008: Validate starting year given on the command line. (#9014)

An invalid starting year causes all sorts of weird behaviour and crashes in map generation.

Now just set the appropriate setting via IConsoleSetSetting so the validation
and, if needed, clamping is performed on the starting year value.

src/openttd.cpp

@@ -455,7 +455,7 @@ struct AfterNewGRFScan : NewGRFScanCallback {
 		/* restore saved music volume */
 		MusicDriver::GetInstance()->SetVolume(_settings_client.music.music_vol);
 
-		if (startyear != INVALID_YEAR) _settings_newgame.game_creation.starting_year = startyear;
+		if (startyear != INVALID_YEAR) IConsoleSetSetting("game_creation.starting_year", startyear);
 		if (generation_seed != GENERATE_NEW_SEED) _settings_newgame.game_creation.generation_seed = generation_seed;
 
 		if (dedicated_host != nullptr) {