Fix #12236: Ship pathfinder causes crash when ship is already at destination

[Kuhnovic]

Motivation / Problem

Fixes #12236

Description

A call to chooseShipTrack is pointless when already at the destination, but it can happen in certain edge cases. Whether these situations should occur in the first place is debatable, and #11862 is an example of such a situation. However, the PF should be able to deal with such strange requests. It certainly shouldn't crash.

I opted for just returning a single random trackdir that can be reached, or INVALID_DIR otherwise, which in turn causes the ship to turn around. This should hopefully get nudge the ship out of the current situation and find a correct path ont he next PF call.

I also decided to not report the ship as lost (by not setting path_found = false), since the destination is technically found and the ship is not really lost. It's a strange situation so the solution is maybe a little strange as well.

Limitations

While this will make the PF more robust, the issue causing these situations is not fixed.

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