media: dvb-usb-v2: ec168: fix null-ptr-deref in ec168_i2c_xfer()

In ec168_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.
If accessing msg[i].buf[0] without sanity check, null pointer 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()")

Link: https://lore.kernel.org/linux-media/20230313085853.3252349-1-harperchen1110@gmail.comSigned-off-by: Wei Chen <harperchen1110@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

drivers/media/usb/dvb-usb-v2/ec168.c

@@ -115,6 +115,10 @@ static int ec168_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msg[],
 	while (i < num) {
 		if (num > i + 1 && (msg[i+1].flags & I2C_M_RD)) {
 			if (msg[i].addr == ec168_ec100_config.demod_address) {
+				if (msg[i].len < 1) {
+					i = -EOPNOTSUPP;
+					break;
+				}
 				req.cmd = READ_DEMOD;
 				req.value = 0;
 				req.index = 0xff00 + msg[i].buf[0]; /* reg */
@@ -131,6 +135,10 @@ static int ec168_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msg[],
 			}
 		} else {
 			if (msg[i].addr == ec168_ec100_config.demod_address) {
+				if (msg[i].len < 1) {
+					i = -EOPNOTSUPP;
+					break;
+				}
 				req.cmd = WRITE_DEMOD;
 				req.value = msg[i].buf[1]; /* val */
 				req.index = 0xff00 + msg[i].buf[0]; /* reg */
@@ -139,6 +147,10 @@ static int ec168_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msg[],
 				ret = ec168_ctrl_msg(d, &req);
 				i += 1;
 			} else {
+				if (msg[i].len < 1) {
+					i = -EOPNOTSUPP;
+					break;
+				}
 				req.cmd = WRITE_I2C;
 				req.value = msg[i].buf[0]; /* val */
 				req.index = 0x0100 + msg[i].addr; /* I2C addr */