exc_get_categories: fix variable check typo

We should check that rv is 0 (argument parse failure) rather than rc
(the actual lcb error code received).

Change-Id: I98620fe10a1ae734da242e796d693688a174b4e8
Reviewed-on: http://review.couchbase.org/37080
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>