Decref iterator if handling artihemtic key/value fails

Change-Id: I0239ec0d4befd1474fe538d84b4ae8953406eb8b
Reviewed-on: http://review.couchbase.org/26660
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>