BUG 0005991 Dwarf tries to build wall standing on construction site

This is a sometimes repeatable problem. There is a wall I have designated to be built on the southwest tower. I'm not sure exactly what is the trigger for the dwarf to stand on the wall he's building: it might depend on where he is standing when he accepts the job. But once he does, and the job gets cancelled, un-suspending the job reliably repeats the problem.

Vanilla DF 0.34.11 except for the tileset.