media: dw2102: Fix null-ptr-deref in dw2102_i2c_transfer()

media: dw2102: Fix null-ptr-deref in dw2102_i2c_transfer()

In dw2102_i2c_transfer, 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 dw2102_i2c_transfer. 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 950e252cb469
("[media] dw2102: limit messages to buffer size")

Signed-off-by: Zhang Shurong <zhang_shurong@foxmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>