MB-11104 Don't return -1 as high seq num for the empty checkpoint

Change-Id: If6abc17a42e0403b90cfb6774c87f7bc37ecc9f5
Reviewed-on: http://review.couchbase.org/37463
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>