media: af9005: Fix null-ptr-deref in af9005_i2c_xfer

In af9005_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 af9005_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 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/af9005.c

@@ -422,6 +422,10 @@ static int af9005_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msg[],
 		if (ret == 0)
 			ret = 2;
 	} else {
+		if (msg[0].len < 2) {
+			ret = -EOPNOTSUPP;
+			goto unlock;
+		}
 		/* write one or more registers */
 		reg = msg[0].buf[0];
 		addr = msg[0].addr;
@@ -431,6 +435,7 @@ static int af9005_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msg[],
 			ret = 1;
 	}
 
+unlock:
 	mutex_unlock(&d->i2c_mutex);
 	return ret;
 }