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.

doc/fixes3-7-0.txt

@@ -1684,6 +1684,8 @@ paranoid_confirm:Autoall for menustyle:full has the test handling backward and
 applying a wielded lump of royal jelly but not picking something to rub it on
 	while other lumps of jelly were quivered would trigger a panic,
 	"addinv: null obj after quiver merge" (like applying lit potion of oil)
+ceiling hider could become hidden when there was no ceiling, triggering a
+	warning if sanity_check was On
 
 
 Fixes to 3.7.0-x Platform and/or Interface Problems Exposed Via git Repository

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;