Fix bug in warmup where open checkpoint is set incorrectly

If the last persisted checkpoint is 0 then this will cause
underflow in the last persisted checkpoint.

Change-Id: I6b8ea20547fff28353042979b9c750c39b2f76f7
Reviewed-on: http://review.couchbase.org/33050
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>