fix prompt string overflow in lift_object()

fix prompt string overflow in lift_object()
<email deleted> wrote:
> The game crashed badly when I made some experiments with items
> with very long names:
>
> You have much trouble lifting a blessed greased thoroughly rusty  >thoroughly corroded +3 plate mail named terribly long killer longer than my
>ong long-worm called long.  Continue? [ynq] (q)

  tty_yn_function(const char * 0x0012fa50,
      const char * 0x00572ddc _ynqchars, char 113) line 379 + 6 bytes
  lift_object(obj * 0x009e8970, obj * 0x00000000,
      long * 0x0012fcd0, char 0) line 1131 + 20 bytes
  pickup_object(obj * 0x009e8970, long 1, char 0) line 1258 + 19 bytes
  pickup(int 0) line 474 + 28 bytes
  dopickup() line 1853 + 11 bytes
  rhack(char * 0x005c0d50 in_line) line 1908 + 3 bytes
  moveloop() line 406 + 7 bytes
  main(int 3, char * * 0x009e2ac0) line 102