Disallow exiting pilot selection without selecting a pilot

[Lgt2x]

Pull Request Type

• [ ] GitHub Workflow changes
• [ ] Documentation or Wiki changes
• [ ] Build and Dependency changes
• [x] Runtime changes
  • [ ] Render changes
  • [ ] Audio changes
  • [ ] Input changes
  • [ ] Network changes
  • [ ] Other changes

Description

Message box did not display correctly when cancelling (ESC) the pilot selection menu, allowing entering the game without a valid pilot

Related Issues

Completes #598

Same vein as #594

Screenshots (if applicable)

Checklist

• [ ] I have tested my changes locally and verified that they work as intended.
• [ ] I have documented any new or modified functionality.
• [ ] I have reviewed the changes to ensure they do not introduce any unnecessary complexity or duplicate code.
• [ ] I understand that by submitting this pull request, I am agreeing to license my contributions under the project's license.

Additional Comments

[Lgt2x]

maybe cfexist should return CFES_NOT_FOUND when filename is null, needs testing to see if it does not break anything else

[winterheart]

Tested on Linux, no issues found.