PYCBC-128: Set instance to NULL on lcb_create failure

Change-Id: I0d6105dc365f9e144519d2f2041de6ac94d18b2b
Reviewed-on: http://review.couchbase.org/26830
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>