USB: cdc-acm: fix potential null-pointer dereference

Must check return value of tty_port_tty_get.

Cc: stable <stable@kernel.org>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/class/cdc-acm.c

@@ -533,6 +533,8 @@ static void acm_softint(struct work_struct *work)
 	if (!ACM_READY(acm))
 		return;
 	tty = tty_port_tty_get(&acm->port);
+	if (!tty)
+		return;
 	tty_wakeup(tty);
 	tty_kref_put(tty);
 }