Pathfinding: Prevent NPCs to jump to open air

[o175]

Pathfinding: Prevent NPCs falling while trying to navigate through open air tile

Summary

Bugfixes "Pathfinding: Avoid NPCs falling down to open air"

Purpose of change

NPCs would try to go through tiles with no floor and fell down hurting themselves. I noticed this when navigating two-storey locations like mansions.

Describe the solution

Add check to npc::get_path_avoid();

Describe alternatives you've considered

Considered modifying the sees_dangerous_field function. But that function does not seem appropriate for this logic

Testing

Created a save with a hole in a roof to test this exact behavior. Before the change NPC was falling down in the hole while following PC. After he started to go around the hole. I provide the save If you want to test it yourself.
Commerce City-trimmed.tar.gz
Autotests are green.

Additional context