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: I412e2b515404537713ed3f5e64aaf67d3f4b56cf
Signed-off-by: Sandya Chikka <sandyachikka@codeaurora.org>