Stonesense hangs when building on the lowest level of hell

[MacHac]

As the title says, Stonesense consistently hangs when designating a construction on the bottommost layer of hell. Other layers are unaffected. I suspect the issue is caused by some function only accounting for layers at or above z level 1, with the bottom of hell being z level 0. Stonesense renders the lowest level of hell just fine when you're not trying to build on it, and there is no hang if stonesense is loaded after the construction is finished, suggesting that the issue lies in the code to create the designation outlines.