MB-10137 zero-initialize compaction_ctx

Change-Id: Ic526dc0313755ba7ebe40cad2b5544a26f225bcf
Reviewed-on: http://review.couchbase.org/33300
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>