media: dvb-usb-v2: gl861: Fix null-ptr-deref in gl861_i2c_master_xfer

In gl861_i2c_master_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 gl861_i2c_master_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 0ed554fd
("media: dvb-usb: az6027: fix null-ptr-deref in az6027_i2c_xfer()")
Signed-off-by: Zhang Shurong <zhang_shurong@foxmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

drivers/media/usb/dvb-usb-v2/gl861.c

@@ -120,7 +120,7 @@ static int gl861_i2c_master_xfer(struct i2c_adapter *adap, struct i2c_msg msg[],
 	} else if (num == 2 && !(msg[0].flags & I2C_M_RD) &&
 		   (msg[1].flags & I2C_M_RD)) {
 		/* I2C write + read */
-		if (msg[0].len > 1 || msg[1].len > sizeof(ctx->buf)) {
+		if (msg[0].len != 1 || msg[1].len > sizeof(ctx->buf)) {
 			ret = -EOPNOTSUPP;
 			goto err;
 		}