mklev() set xxstairs_room pointers back to NULL upon completion.

When mklev() is called multiple times, previous state stored in the
xxstairs_room pointers can be mistakenly used when making decisions about
the new level being constructed.  This caused non-deterministic level
creation behavior when replaying from a snapshot.

src/mklev.c

@@ -1016,6 +1016,10 @@ mklev()
     for (ridx = 0; ridx < SIZE(rooms); ridx++)
         rooms[ridx].orig_rtype = rooms[ridx].rtype;
 
+    dnstairs_room = NULL;
+    upstairs_room = NULL;
+    sstairs_room = NULL;
+
     reseed_random(rn2);
     reseed_random(rn2_on_display_rng);
 }