MB-7629: Replace some unnecessary use of sized_buf

Change-Id: Iaea329566d9d64c4377af65f6695180c28e5a94d
Reviewed-on: http://review.couchbase.org/26954
Tested-by: Fulu Li <fulu@couchbase.com>
Reviewed-by: Aaron Miller <apage43@ninjawhale.com>