i2c-msm-v2: Add NULL pointer check

Add NULL pointer check for client passed pointer to
make sure there are no crashes in the i2c driver if
the client driver passes the NULL pointer.

Change-Id: I91f0d7ec2c9dcb38eea1c1564a47e8207b7d038e
Signed-off-by: Sandya Chikka <sandyachikka@codeaurora.org>