[media] dvb: si21xx: buffer overflow in si21_writeregs()

"len" is user controlled and can be up to 255.  Anything more than 59
will cause a buffer overflow so we need to add a test for that.
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/dvb-frontends/si21xx.c

@@ -236,6 +236,9 @@ static int si21_writeregs(struct si21xx_state *state, u8 reg1,
 				.len = len + 1
 	};
 
+	if (len > sizeof(buf) - 1)
+		return -EINVAL;
+
 	msg.buf[0] =  reg1;
 	memcpy(msg.buf + 1, data, len);