impossible "ceiling hider hiding without ceiling"

Reported by a hardfought user, a ceiling_hider monster hid on the
Astral level, triggering a sanity check warning that gets repeated
each move until the hider comes out of hiding.  Normally sanity_check
can only be set in wizard mode, but hardfought must force it on for
to-be-3.7.

I was able to reproduce it before this fix and unable to do so
afterward, but there is a random factor involved hence no guarantees.
I think that lack of ceiling for Astral is recent, but this could
have happened on other levels which lack a ceiling.  I didn't try to
figure out whether it might happen with 3.6.x, just put the fixes
entry in the "exposed by git" (found in code not yet released, that
is) section.

src/mon.c

@@ -4214,6 +4214,9 @@ restrap(struct monst *mtmp)
         /* can't hide while trapped except in pits */
         || (mtmp->mtrapped && (t = t_at(mtmp->mx, mtmp->my)) != 0
             && !is_pit(t->ttyp))
+        /* can't hide on ceiling if there isn't one */
+        || (ceiling_hider(mtmp->data) && !has_ceiling(&u.uz))
+        /* won't hide when adjacent to hero */
         || (sensemon(mtmp) && next2u(mtmp->mx, mtmp->my)))
         return FALSE;