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

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 0ed554fd769a ("media: dvb-usb: az6027: fix null-ptr-deref in az6027_i2c_xfer()")

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