fix pull request #527 - cursed levitation

The stair handling reorganization changed drinking a cursed potion
of levitation to only check whether stairs up existed on the level
instead of whether the hero drank the potion while at their spot.
That resulted in always attempting to go up and then getting "you
can't go up here" when not at stairs instead of the intended "you
hit your head on the ceiling".

Fixes #527

src/potion.c

@@ -1033,13 +1033,15 @@ peffects(struct obj *otmp)
             g.potion_nothing++;
 
         if (otmp->cursed) {
+            stairway *stway;
+
             /* 'already levitating' used to block the cursed effect(s)
                aside from ~I_SPECIAL; it was not clear whether that was
                intentional; either way, it no longer does (as of 3.6.1) */
             HLevitation &= ~I_SPECIAL; /* can't descend upon demand */
             if (BLevitation) {
                 ; /* rising via levitation is blocked */
-            } else if (stairway_find_dir(TRUE)) {
+            } else if ((stway = stairway_at(u.ux, u.uy)) != 0 && stway->up) {
                 (void) doup();
                 /* in case we're already Levitating, which would have
                    resulted in incrementing 'nothing' */