Fix: map random y-placement used its width instead of height

Not sure how long this has existed without triggering any issues, but
when I was testing out a themed room wider than it was tall, I ran into
rn2-of-a-negative-number impossibles. Traced it to here, where it was
trying to subtract the width of the mapfrag from ROWNO to figure out
which y-value it should place the map on. The correct behavior is to
subtract the height of the mapfrag.

src/sp_lev.c

@@ -6133,7 +6133,7 @@ TODO: gc.coder->croom needs to be updated
                     if (y < 1)
                         y = 1;
                 } else {
-                    y = rn2(ROWNO - mf->wid);
+                    y = rn2(ROWNO - mf->hei);
                 }
             }
         }