Make sure the TTY has not disappeared out from under us before calling

ttydevsw_outwakeup().  This should fix panics which occur after remote
login sessions timeout during moderate TTY activity.  An example of
where this might occur is where a pending write to the terminal is
occurring while sshd(8) is shutting down the TTY after a TCP timeout.

Submitted by:	ed

sys/kern/tty_ttydisc.c

@@ -539,8 +539,9 @@ ttydisc_write(struct tty *tp, struct uio *uio, int ioflag)
 		} while (oblen > 0);
 	}
 
-done:	
-	ttydevsw_outwakeup(tp);
+done:
+	if (!tty_gone(tp))
+		ttydevsw_outwakeup(tp);
 
 	/*
 	 * Add the amount of bytes that we didn't process back to the