Reinforcing walls does not update creature pathfinding

[lokrok]

It was discovered in #3724 that if a creature is currently on a path that gets updated somehow by reinforcing walls, the creature will get stuck without realizing that the path is no longer valid. This happens on both Imp reinforcement and via the Make Safe special. Interrupting the creatures action in anyway will make it pathfind properly.

Reproduce steps

1. Make or find a map where a path can be dug out in which the corner is water (and the earth is rounded near it). The map given in #3724 is a good test.
2. Have your imps do a task beyond the corner so that they traverse through it.
3. When your imps head towards the corner, fortify the neighboring earth to seal the open corner.
4. The imps should get stuck in the corner, remaining on the invalid pathfind. Imps that start navigating after the walls are fortified close will navigate correctly (or not navigate at all if there is no valid path).
5. Slapping or updating imps stuck in the corner in any way should fix their path.