Fix compile error on MacOS.
Fix compile error on MacOS.

Summary:
In a release build, a member was not being accessed. This
member was only being accessed in a debug build. We now add an accessor
function for this member and the buid succeeds.

Test Plan: build release/unity/debug on linux/mac

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D34035

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ality/rocksdb,vashstorm/rocksdb,JackLian/rocksdb,kaschaeffer/rocksdb,tsheasha/rocksdb,skunkwerks/rocksdb,fengshao0907/rocksdb,biddyweb/rocksdb,kaschaeffer/rocksdb,hobinyoon/rocksdb,jalexanderqed/rocksdb,bbiao/rocksdb,virtdb/rocksdb,norton/rocksdb,wlqGit/rocksdb,wskplho/rocksdb,NickCis/rocksdb,rDSN-Projects/rocksdb.replicated,facebook/rocksdb,hobinyoon/rocksdb,NickCis/rocksdb,facebook/rocksdb,biddyweb/rocksdb,tsheasha/rocksdb,luckywhu/rocksdb,bbiao/rocksdb,anagav/rocksdb,zhangpng/rocksdb,wlqGit/rocksdb,Vaisman/rocksdb,JoeWoo/rocksdb,virtdb/rocksdb,wenduo/rocksdb,tsheasha/rocksdb,siddhartharay007/rocksdb,zhangpng/rocksdb,virtdb/rocksdb,biddyweb/rocksdb,tizzybec/rocksdb,skunkwerks/rocksdb,wat-ze-hex/rocksdb,fengshao0907/rocksdb,OverlordQ/rocksdb,ryneli/rocksdb,flabby/rocksdb,tizzybec/rocksdb,zhangpng/rocksdb,hobinyoon/rocksdb,wat-ze-hex/rocksdb,Applied-Duality/rocksdb,IMCG/RcoksDB,Andymic/rocksdb,IMCG/RcoksDB,vmx/rocksdb,JohnPJenkins/rocksdb,IMCG/RcoksDB,norton/rocksdb,bbiao/rocksdb,JoeWoo/rocksdb,zhangpng/rocksdb,alihalabyah/rocksdb,Vaisman/rocksdb