Fix #9966: workaround a VS2022 optimisation bug

[glx22]

Motivation / Problem

Starting with MSVC 19.30, there is an optimisation bug silently converting a *q++ = '\r'; into q++;.
A bug report about it is still open.

Description

Applying the workaround to get our code to do what we expect.

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, gs_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