crash fix: shopkeeper re-entering shop

- when a shopkeeper leaves the shop to chase the player, and the player
enters the shop, bill_p is set to an unusual value.  bill_p needs to be set
back to a valid value if the shopkeeper re-enters the shop.
- Also, the u.ushops state needs to be updated when a shop becomes tended
again if the player is in the shop.
- introduce a new after_shk_move function to handle this

src/monmove.c

@@ -1194,6 +1194,9 @@ postmov:
 			(is_pool(mtmp->mx, mtmp->my) && !Is_waterlevel(&u.uz));
 		newsym(mtmp->mx, mtmp->my);
 	    }
+	    if (mtmp->isshk) {
+		after_shk_move(mtmp);
+	    }
 	}
 	return(mmoved);
 }

src/shk.c

@@ -891,6 +891,7 @@ register boolean killkops;
 #endif
 		pacify_guards();
 	}
+	after_shk_move(shkp);
 }
 
 STATIC_OVL boolean
@@ -3257,8 +3258,24 @@ register struct monst *shkp;
 		}
 	}
 
-	return(move_special(shkp,inhishop(shkp),
-			    appr,uondoor,avoid,omx,omy,gx,gy));
+	z = move_special(shkp,inhishop(shkp),appr,uondoor,avoid,omx,omy,gx,gy);
+	if (z > 0) after_shk_move(shkp);
+
+	return z;
+}
+
+/* called after shopkeeper moves, in case the move causes re-entry into shop */
+void
+after_shk_move(shkp)
+struct monst *shkp;
+{
+	struct eshk *eshkp = ESHK(shkp);
+
+	if (eshkp->bill_p == (struct bill_x *) -1000 && inhishop(shkp)) {
+	    /* reset bill_p, need to re-calc player's occupancy too */
+	    eshkp->bill_p = &eshkp->bill[0];
+	    check_special_room(FALSE);
+	}
 }
 
 #endif /*OVL3*/