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

doc/fixes37.0

@@ -705,6 +705,8 @@ lighting for baalz level defaulted to random and could end up being lit
 when using 'O' to set menu color patterns, the reminder message that the
 	boolean menucolors option needs to be set to True for them to work
 	was sometimes given even when already True
+stair revamp unintentionally resulted in cursed potion of levitation no longer
+	causing hero to hit head on ceiling
 
 curses: 'msg_window' option wasn't functional for curses unless the binary
 	also included tty support

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' */