tty: USB: serial/mct_u232, fix tty refcnt

Stanse found a tty refcnt leak in read_int_callback. In fact
it's handled wrong altogether. tty_port_tty_get can return NULL
and it's not checked in that manner.

Fix that by checking the tty_port_tty_get retval and put tty kref
properly.

http://stanse.fi.muni.cz/Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/mct_u232.c

@@ -566,10 +566,13 @@ static void mct_u232_read_int_callback(struct urb *urb)
 	 * Work-a-round: handle the 'usual' bulk-in pipe here
 	 */
 	if (urb->transfer_buffer_length > 2) {
-		tty = tty_port_tty_get(&port->port);
 		if (urb->actual_length) {
-			tty_insert_flip_string(tty, data, urb->actual_length);
-			tty_flip_buffer_push(tty);
+			tty = tty_port_tty_get(&port->port);
+			if (tty) {
+				tty_insert_flip_string(tty, data,
+						urb->actual_length);
+				tty_flip_buffer_push(tty);
+			}
 			tty_kref_put(tty);
 		}
 		goto exit;