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

In the Linux kernel, the following vulnerability has been resolved:

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 0ed554fd769a

("media: dvb-usb: az6027: fix null-ptr-deref in az6027_i2c_xfer()")