tty: pty, remove BUG_ON from pty_close

tty->ops->close is always called with a valid tty, so the BUG_ON cannot
trigger.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210105120239.28031-5-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/pty.c

@@ -45,7 +45,6 @@ static DEFINE_MUTEX(devpts_mutex);
 
 static void pty_close(struct tty_struct *tty, struct file *filp)
 {
-	BUG_ON(!tty);
 	if (tty->driver->subtype == PTY_TYPE_MASTER)
 		WARN_ON(tty->count > 1);
 	else {