MB-9147: lseek can return 0 if the file has no length

This will avoid asserting incorrectly.

Change-Id: Ibca0d69782f753e0dbb16652d5591a21e514c9f9
Reviewed-on: http://review.couchbase.org/29048
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>