PYCBC-111: Fix leak on bad arguments

Change-Id: I43e5ed18641985b5af2bd0f8f022e53818d20c0b
Reviewed-on: http://review.couchbase.org/26289
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>