Defend the i2c refcount calls against NULL pointers, as is important
(and conventional) for such calls.  Note that none of the current
callers of i2c_use_client() use its return value.

[JD: I hate this but apparently all the other subsystems do it so...]
Signed-off-by: NDavid Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: NJean Delvare <khali@linux-fr.org>