#2791 [cdo, 8622115] Ziggurat levels generated incorrectly

[N78291]

The first level was room and corridor style and had up and down escape hatches. Entering the ziggurat produced several

"
Lua error: /crawl-svn/dat/clua/ziggurat.lua:733: bad argument #1 to
'fill_area' (dgn.mtmap expected, got number)
"
and generated many entry milestones.

[Jude Brown]

Enne recently changed the way that the dungeon builder functions work, and it appears to be something related to this, so assigning to Enne.