i2c: add sanity check to i2c_put_adapter

i2c_put_adapter dereferences i2c_adapter pointer passed without check
for NULL. This adds a check for non-NULL pointer to allow i2c_put_adapter
called with NULL and behave the same way i2c_release_client does already.
It allows to simplify drivers where you need to release the adapter
during probe failures.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>