serial: access after NULL check in uart_flush_buffer()

I noticed that

  static void uart_flush_buffer(struct tty_struct *tty)
  {
  	struct uart_state *state = tty->driver_data;
  	struct uart_port *port = state->port;
  	unsigned long flags;

  	/*
  	 * This means you called this function _after_ the port was
  	 * closed.  No cookie for you.
  	 */
  	if (!state || !state->info) {
  		WARN_ON(1);
  		return;
  	}

is too late for checking state != NULL.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>