MB-100 iterate_docinfos requires a positive number of input keys

Change-Id: I4e922bd83b223bb1d54ab561701eeb91afe69414
Reviewed-on: http://review.couchbase.org/26798
Reviewed-by: Fulu Li <fulu@couchbase.com>
Tested-by: Aaron Miller <apage43@ninjawhale.com>