bug#1052: segfault when resuming emacsclient -t in an xterm

[Chong Yidong]

> The problem is that after the cited change
> `xterm-remove-modify-other-keys' calls `terminal-live-p' (it was
> previously using `frame-live-p') before calling
> `send-string-to-terminal'.
>
> `terminal-live-p' does not return false when tty->output is NULL --->
> KABOOM.

Looks like it's just a matter of checking tty->output, then.  WDYT?

*** trunk/src/dispnew.c.~1.419.~        2008-09-28 16:09:43.000000000 -0400
--- trunk/src/dispnew.c 2008-09-30 13:02:21.000000000 -0400
***************
*** 6470,6477 ****
        fwrite (SDATA (string), 1, SBYTES (string), tty->termscript);
        fflush (tty->termscript);
      }
!   fwrite (SDATA (string), 1, SBYTES (string), tty->output);
!   fflush (tty->output);
    UNBLOCK_INPUT;
    return Qnil;
  }
--- 6470,6480 ----
        fwrite (SDATA (string), 1, SBYTES (string), tty->termscript);
        fflush (tty->termscript);
      }
!   if (tty->output)
!     {
!       fwrite (SDATA (string), 1, SBYTES (string), tty->output);
!       fflush (tty->output);
!     }
    UNBLOCK_INPUT;
    return Qnil;
  }