lcbio_CTX: Don't write empty ringbuffer IOV

Change-Id: I4446f8f6577d98d9b04e26e8184f0ece4c3bf518
Reviewed-on: http://review.couchbase.org/37333
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>