media: dvb-usb: digitv: fix null-ptr-deref in digitv_i2c_xfer()

In digitv_i2c_xfer, msg is controlled by user. When msg[i].buf
is null and msg[i].len is zero, former checks on msg[i].buf would be
passed. Malicious data finally reach digitv_i2c_xfer. If accessing
msg[i].buf[0] without sanity check, null ptr deref would happen. We add
check on msg[i].len to prevent crash.

Similar commit:
commit 0ed554f ("media: dvb-usb: az6027: fix null-ptr-deref in az6027_i2c_xfer()")

Link: https://lore.kernel.org/linux-media/20230313095008.1039689-1-harperchen1110@gmail.com
Signed-off-by: Wei Chen <harperchen1110@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

Author: harperchen

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

@@ -63,6 +63,10 @@ static int digitv_i2c_xfer(struct i2c_adapter *adap,struct i2c_msg msg[],int num
 		warn("more than 2 i2c messages at a time is not handled yet. TODO.");
 
 	for (i = 0; i < num; i++) {
+		if (msg[i].len < 1) {
+			i = -EOPNOTSUPP;
+			break;
+		}
 		/* write/read request */
 		if (i+1 < num && (msg[i+1].flags & I2C_M_RD)) {
 			if (digitv_ctrl_msg(d, USB_READ_COFDM, msg[i].buf[0], NULL, 0,