Fix shopkeeper Kop summons causing segfault

... if it happened on a level with no downstairs, such as the tourist quest.

include/extern.h

@@ -3112,6 +3112,7 @@ extern void nethack_enter_windows(void);
 extern void amulet(void);
 extern int mon_has_amulet(struct monst *);
 extern int mon_has_special(struct monst *);
+extern void choose_stairs(xchar *, xchar *, boolean);
 extern int tactics(struct monst *);
 extern boolean has_aggravatables(struct monst *);
 extern void aggravate(void);

src/shk.c

@@ -344,14 +344,9 @@ call_kops(register struct monst* shkp, register boolean nearshop)
 
     {
         coord mm;
-        stairway *stway = g.stairs;
+        xchar sx = 0, sy = 0;
 
-        while (stway) {
-            if (!stway->isladder && !stway->up
-                && stway->tolev.dnum == u.uz.dnum)
-                break;
-            stway = stway->next;
-        }
+        choose_stairs(&sx, &sy, TRUE);
 
         if (nearshop) {
             /* Create swarm around you, if you merely "stepped out" */
@@ -365,9 +360,11 @@ call_kops(register struct monst* shkp, register boolean nearshop)
         if (flags.verbose)
             pline_The("Keystone Kops are after you!");
         /* Create swarm near down staircase (hinders return to level) */
-        mm.x = stway->sx;
-        mm.y = stway->sy;
-        makekops(&mm);
+        if (isok(sx, sy)) {
+            mm.x = sx;
+            mm.y = sy;
+            makekops(&mm);
+        }
         /* Create swarm near shopkeeper (hinders return to shop) */
         mm.x = shkp->mx;
         mm.y = shkp->my;

src/wizard.c

@@ -17,7 +17,6 @@ static struct obj *on_ground(short);
 static boolean you_have(int);
 static unsigned long target_on(int, struct monst *);
 static unsigned long strategy(struct monst *);
-static void choose_stairs(xchar *, xchar *);
 
 /* adding more neutral creatures will tend to reduce the number of monsters
    summoned by nasty(); adding more lawful creatures will reduce the number
@@ -311,12 +310,12 @@ strategy(struct monst *mtmp)
     return dstrat;
 }
 
-static void
-choose_stairs(xchar *sx, xchar *sy)
+void
+choose_stairs(xchar *sx, xchar *sy, boolean dir)
 {
     xchar x = 0, y = 0;
     stairway *stway = g.stairs;
-    boolean stdir = !builds_up(&u.uz);
+    boolean stdir = dir && !builds_up(&u.uz);
 
     if ((stway = stairway_find_type_dir(FALSE, stdir)) != 0) {
         x = stway->sx;
@@ -335,7 +334,7 @@ choose_stairs(xchar *sx, xchar *sy)
         }
     }
 
-    if (x && y) {
+    if (isok(x, y)) {
         *sx = x;
         *sy = y;
     }
@@ -356,7 +355,7 @@ tactics(struct monst *mtmp)
     case STRAT_HEAL: /* hide and recover */
         mx = mtmp->mx, my = mtmp->my;
         /* if wounded, hole up on or near the stairs (to block them) */
-        choose_stairs(&sx, &sy);
+        choose_stairs(&sx, &sy, FALSE);
         mtmp->mavenge = 1; /* covetous monsters attack while fleeing */
         if (In_W_tower(mx, my, &u.uz)
             || (mtmp->iswiz && !sx && !mon_has_amulet(mtmp))) {