MB-9307: Don't rollback if partition didn't exist

Don't roll back if a partition doesn't exist in a
previous header (or has sequence number 0).

Change-Id: Idb99d557aa27339989d3e5c2b9c3a28c158e16c9
Reviewed-on: http://review.couchbase.org/30021
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>