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);
 }