MB-11352: Fix base size calculation for mutation/deletion packets

Change-Id: Ife64e0440c9662eb0ccb3c84806718124b168fd3
Reviewed-on: http://review.couchbase.org/38031
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>