tty: handle NULL parameters in free_tty_struct()

We sometimes pass NULL pointers to free_tty_struct().  One example where
it can happen is in the error handling code in pty_common_install().
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -181,6 +181,8 @@ struct tty_struct *alloc_tty_struct(void)
 
 void free_tty_struct(struct tty_struct *tty)
 {
+	if (!tty)
+		return;
 	if (tty->dev)
 		put_device(tty->dev);
 	kfree(tty->write_buf);