[BUG] Basher can fall through builder staircase

[namida]

To reproduce:
1. Let a builder build at least two steps
2. Assign a basher to a lemming who is standing on one step, and on the next frame will move upwards onto the next step

The result is that the lemming will have the basher animation for 1 frame, before falling through the bridge (without damaging it).

[namida]

Fixed in commit 88f5b37.