Impossible Dungeon Bug

[surrsurus]

Describe the bug
In some dungeons the stairs may spawn in unreachable locations.

To Reproduce
Steps to reproduce the behavior:

1. Start game
2. Toggle FOV off
3. Regenerate floors until one spawns with unreachable stairs, either up or down

Expected behavior
All dungeons should be able to be traversable, that is, a walkable path from up stair to down stair can be made that does not cross over hazards.

Additional context
Can probably fixed by on first map generation, check if a solid path can be made from up stair to down stair with tcod's built in A* algorithm.