Fix #11164: Duplicate town names when using the many random towns function

[JGRennison]

Motivation / Problem

#11164

Description

Fix #11164

Limitations

N/A

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

[2TallTyler]

This may leave a possibility of duplicate names if the command cannot generate towns on its initial try and moves on to the more aggressive attempt below (all in the same function), since the name list is cleared first and then not even checked in the aggressive attempt.

That should probably be verified, and if so, handled somehow, even if it's just by bailing out on L2281 if more than zero towns already exist.

[JGRennison]

The "aggressive" attempt below does not use town_names as a parameter to VerifyTownName, so there is not a risk of duplicate town names in this case.

[2TallTyler]

Ah, I see that VerifyTownName() iterates through existing towns on its own if not provided a list.