[media] digitv: limit messages to buffer size

Return an error rather than memcpy()ing beyond the end of the buffer.
Internal callers use appropriate sizes, but digitv_i2c_xfer may not.
Signed-off-by: Alyssa Milburn <amilburn@zall.org>
Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/usb/dvb-usb/digitv.c

@@ -33,6 +33,9 @@ static int digitv_ctrl_msg(struct dvb_usb_device *d,
 
 	wo = (rbuf == NULL || rlen == 0); /* write-only */
 
+	if (wlen > 4 || rlen > 4)
+		return -EIO;
+
 	memset(st->sndbuf, 0, 7);
 	memset(st->rcvbuf, 0, 7);