i2c: dev: prevent ZERO_SIZE_PTR deref in i2cdev_ioctl_rdwr()

i2cdev_ioctl_rdwr() allocates i2c_msg.buf using memdup_user(), which
returns ZERO_SIZE_PTR if i2c_msg.len is zero.

Currently i2cdev_ioctl_rdwr() always dereferences the buf pointer in case
of I2C_M_RD | I2C_M_RECV_LEN transfer. That causes a kernel oops in
case of zero len.

Let's check the len against zero before dereferencing buf pointer.

This issue was triggered by syzkaller.

Signed-off-by: Alexander Popov <alex.popov@linux.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
[wsa: use '< 1' instead of '!' for easier readability]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

drivers/i2c/i2c-dev.c

@@ -280,7 +280,7 @@ static noinline int i2cdev_ioctl_rdwr(struct i2c_client *client,
  */
  if (msgs[i].flags & I2C_M_RECV_LEN) {
  if (!(msgs[i].flags & I2C_M_RD) ||
- msgs[i].buf[0] < 1 ||
+ msgs[i].len < 1 || msgs[i].buf[0] < 1 ||
  msgs[i].len < msgs[i].buf[0] +
  I2C_SMBUS_BLOCK_MAX) {
  res = -EINVAL;