media: az6007: Fix null-ptr-deref in az6007_i2c_xfer()

In az6007_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 az6007_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-v2/az6007.c

@@ -788,6 +788,10 @@ static int az6007_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msgs[],
 			if (az6007_xfer_debug)
 				printk(KERN_DEBUG "az6007: I2C W addr=0x%x len=%d\n",
 				       addr, msgs[i].len);
+			if (msgs[i].len < 1) {
+				ret = -EIO;
+				goto err;
+			}
 			req = AZ6007_I2C_WR;
 			index = msgs[i].buf[0];
 			value = addr | (1 << 8);
@@ -802,6 +806,10 @@ static int az6007_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msgs[],
 			if (az6007_xfer_debug)
 				printk(KERN_DEBUG "az6007: I2C R addr=0x%x len=%d\n",
 				       addr, msgs[i].len);
+			if (msgs[i].len < 1) {
+				ret = -EIO;
+				goto err;
+			}
 			req = AZ6007_I2C_RD;
 			index = msgs[i].buf[0];
 			value = addr;