USB: serial/kobil_sct, fix potential tty NULL dereference

Make sure that we check the return value of tty_port_tty_get.
Sometimes it may return NULL and we later dereference that.

The only place here is in kobil_read_int_callback, so fix it.
Signed-off-by: NJiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@linux.intel.com>
Cc: stable@kernel.org
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/kobil_sct.c

@@ -372,7 +372,7 @@ static void kobil_read_int_callback(struct urb *urb)
 	}
 
 	tty = tty_port_tty_get(&port->port);
-	if (urb->actual_length) {
+	if (tty && urb->actual_length) {
 
 		/* BEGIN DEBUG */
 		/*