Motivation / Problem

A null pointer dereference occurs when GetIncompatibleRefitOrderIdForAutoreplace is called on a vehicle with no orders, such that the order list is null.
(This also includes autoreplacing of free-wagon chains).

This was introduced in PR #8169.

Description

Handle the case where the order list is null in GetIncompatibleRefitOrderIdForAutoreplace.

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 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