Motivation / Problem

Packet changed a lot 6 months ago, and some conditional code was not updated.

Description

I adapted @JGRennison's fix.
My original version was a direct conversion p->CanReadFromPacket(1) but it's better to properly check the sizes.
I also added RANDOM_DEBUG flag to dedicated build CI check as a future proof measure.

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