Fix db_bench's multireadrandom
Fix db_bench's multireadrandom

Summary: multireadrandom is broken. Fix it

Test Plan: run it and see segfault has gone.

Reviewers: ljin

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17781

msb-at-yahoo/rocksdb,wlqGit/rocksdb,fengshao0907/rocksdb,NickCis/rocksdb,OverlordQ/rocksdb,wskplho/rocksdb,makelivedotnet/rocksdb,ryneli/rocksdb,tsheasha/rocksdb,facebook/rocksdb,wat-ze-hex/rocksdb,sorphi/rocksdb,tsheasha/rocksdb,kaschaeffer/rocksdb,siddhartharay007/rocksdb,wlqGit/rocksdb,Andymic/rocksdb,Applied-Duality/rocksdb,caijieming-baidu/rocksdb,JackLian/rocksdb,RyanTech/rocksdb,JohnPJenkins/rocksdb,ryneli/rocksdb,bbiao/rocksdb,vashstorm/rocksdb,JohnPJenkins/rocksdb,luckywhu/rocksdb,norton/rocksdb,amyvmiwei/rocksdb,geraldoandradee/rocksdb,fengshao0907/rocksdb,amyvmiwei/rocksdb,Applied-Duality/rocksdb,dkorolev/rocksdb,kaschaeffer/rocksdb,caijieming-baidu/rocksdb,virtdb/rocksdb,JohnPJenkins/rocksdb,rDSN-Projects/rocksdb.replicated,sorphi/rocksdb,ryneli/rocksdb,Andymic/rocksdb,IMCG/RcoksDB,tsheasha/rocksdb,SunguckLee/RocksDB,tizzybec/rocksdb,makelivedotnet/rocksdb,msb-at-yahoo/rocksdb,OverlordQ/rocksdb,makelivedotnet/rocksdb,SunguckLee/RocksDB,facebook/rocksdb,ylong/rocksdb,jalexanderqed/rocksdb,fengshao0907/rocksdb,rDSN-Projects/rocksdb.replicated,mbarbon/rocksdb,tschottdorf/rocksdb,alihalabyah/rocksdb,wenduo/rocksdb,skunkwerks/rocksdb,geraldoandradee/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,SunguckLee/RocksDB,siddhartharay007/rocksdb,IMCG/RcoksDB,geraldoandradee/rocksdb,Andymic/rocksdb,wenduo/rocksdb,JackLian/rocksdb,IMCG/RcoksDB,makelivedotnet/rocksdb,alihalabyah/rocksdb,RyanTech/rocksdb,tizzybec/rocksdb,makelivedotnet/rocksdb,JohnPJenkins/rocksdb,skunkwerks/rocksdb,alihalabyah/rocksdb,mbarbon/rocksdb,ylong/rocksdb,luckywhu/rocksdb,wenduo/rocksdb,zhangpng/rocksdb,wlqGit/rocksdb,wlqGit/rocksdb,ylong/rocksdb,caijieming-baidu/rocksdb,tsheasha/rocksdb,RyanTech/rocksdb,makelivedotnet/rocksdb,facebook/rocksdb,dkorolev/rocksdb,vashstorm/rocksdb,luckywhu/rocksdb,luckywhu/rocksdb,hobinyoon/rocksdb,vmx/rocksdb,temicai/rocksdb,luckywhu/rocksdb,kaschaeffer/rocksdb,dkorolev/rocksdb,dkorolev/rocksdb,zhangpng/rocksdb,luckywhu/rocksdb,facebook/rocksdb,flabby/rocksdb,sorphi/rocksdb,jalexanderqed/rocksdb,tizzybec/rocksdb,lgscofield/rocksdb,amyvmiwei/rocksdb,jalexanderqed/rocksdb,alihalabyah/rocksdb,ryneli/rocksdb,OverlordQ/rocksdb,Andymic/rocksdb,anagav/rocksdb,anagav/rocksdb,JoeWoo/rocksdb,geraldoandradee/rocksdb,jalexanderqed/rocksdb,norton/rocksdb,ylong/rocksdb,wskplho/rocksdb,wenduo/rocksdb,dkorolev/rocksdb,geraldoandradee/rocksdb,Vaisman/rocksdb,virtdb/rocksdb,JoeWoo/rocksdb,hobinyoon/rocksdb,tsheasha/rocksdb,wlqGit/rocksdb,tizzybec/rocksdb,skunkwerks/rocksdb,kaschaeffer/rocksdb,wenduo/rocksdb,rDSN-Projects/rocksdb.replicated,ylong/rocksdb,tschottdorf/rocksdb,skunkwerks/rocksdb,JoeWoo/rocksdb,facebook/rocksdb,siddhartharay007/rocksdb,hobinyoon/rocksdb,siddhartharay007/rocksdb,fengshao0907/rocksdb,fengshao0907/rocksdb,virtdb/rocksdb,Applied-Duality/rocksdb,flabby/rocksdb,hobinyoon/rocksdb,wenduo/rocksdb,IMCG/RcoksDB,tschottdorf/rocksdb,Applied-Duality/rocksdb,RyanTech/rocksdb,luckywhu/rocksdb,vashstorm/rocksdb,rDSN-Projects/rocksdb.replicated,vmx/rocksdb,temicai/rocksdb,wenduo/rocksdb,bbiao/rocksdb,jalexanderqed/rocksdb,Andymic/rocksdb,Vaisman/rocksdb,alihalabyah/rocksdb,alihalabyah/rocksdb,geraldoandradee/rocksdb,norton/rocksdb,JackLian/rocksdb,flabby/rocksdb,lgscofield/rocksdb,lgscofield/rocksdb,OverlordQ/rocksdb,ylong/rocksdb,msb-at-yahoo/rocksdb,temicai/rocksdb,Vaisman/rocksdb,JackLian/rocksdb,NickCis/rocksdb,msb-at-yahoo/rocksdb,bbiao/rocksdb,wlqGit/rocksdb,Andymic/rocksdb,vmx/rocksdb,OverlordQ/rocksdb,msb-at-yahoo/rocksdb,vmx/rocksdb,tsheasha/rocksdb,skunkwerks/rocksdb,wat-ze-hex/rocksdb,wskplho/rocksdb,RyanTech/rocksdb,Applied-Duality/rocksdb,tizzybec/rocksdb,mbarbon/rocksdb,norton/rocksdb,anagav/rocksdb,hobinyoon/rocksdb,zhangpng/rocksdb,norton/rocksdb,flabby/rocksdb,NickCis/rocksdb,makelivedotnet/rocksdb,jalexanderqed/rocksdb,bbiao/rocksdb,kaschaeffer/rocksdb,norton/rocksdb,wskplho/rocksdb,biddyweb/rocksdb,biddyweb/rocksdb,wlqGit/rocksdb,rDSN-Projects/rocksdb.replicated,ylong/rocksdb,anagav/rocksdb,vashstorm/rocksdb,biddyweb/rocksdb,JoeWoo/rocksdb,tizzybec/rocksdb,wat-ze-hex/rocksdb,JoeWoo/rocksdb,Applied-Duality/rocksdb,wskplho/rocksdb,dkorolev/rocksdb,siddhartharay007/rocksdb,amyvmiwei/rocksdb,virtdb/rocksdb,sorphi/rocksdb,flabby/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,fengshao0907/rocksdb,Vaisman/rocksdb,virtdb/rocksdb,SunguckLee/RocksDB,anagav/rocksdb,Vaisman/rocksdb,flabby/rocksdb,kaschaeffer/rocksdb,lgscofield/rocksdb,anagav/rocksdb,skunkwerks/rocksdb,skunkwerks/rocksdb,JohnPJenkins/rocksdb,temicai/rocksdb,vmx/rocksdb,zhangpng/rocksdb,mbarbon/rocksdb,norton/rocksdb,tsheasha/rocksdb,jalexanderqed/rocksdb,NickCis/rocksdb,Vaisman/rocksdb,virtdb/rocksdb,temicai/rocksdb,OverlordQ/rocksdb,sorphi/rocksdb,JoeWoo/rocksdb,vmx/rocksdb,caijieming-baidu/rocksdb,NickCis/rocksdb,JohnPJenkins/rocksdb,mbarbon/rocksdb,norton/rocksdb,dkorolev/rocksdb,tizzybec/rocksdb,fengshao0907/rocksdb,tschottdorf/rocksdb,OverlordQ/rocksdb,bbiao/rocksdb,kaschaeffer/rocksdb,alihalabyah/rocksdb,amyvmiwei/rocksdb,RyanTech/rocksdb,tschottdorf/rocksdb,IMCG/RcoksDB,ryneli/rocksdb,mbarbon/rocksdb,caijieming-baidu/rocksdb,vashstorm/rocksdb,flabby/rocksdb,zhangpng/rocksdb,JoeWoo/rocksdb,facebook/rocksdb,Vaisman/rocksdb,anagav/rocksdb,vmx/rocksdb,wskplho/rocksdb,biddyweb/rocksdb,rDSN-Projects/rocksdb.replicated,SunguckLee/RocksDB,rDSN-Projects/rocksdb.replicated,facebook/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,temicai/rocksdb,JackLian/rocksdb,Andymic/rocksdb,zhangpng/rocksdb,msb-at-yahoo/rocksdb,tsheasha/rocksdb,tschottdorf/rocksdb,geraldoandradee/rocksdb,lgscofield/rocksdb,facebook/rocksdb,hobinyoon/rocksdb,wat-ze-hex/rocksdb,siddhartharay007/rocksdb,hobinyoon/rocksdb,amyvmiwei/rocksdb,virtdb/rocksdb,zhangpng/rocksdb,hobinyoon/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,biddyweb/rocksdb,biddyweb/rocksdb,temicai/rocksdb,siddhartharay007/rocksdb,wenduo/rocksdb,caijieming-baidu/rocksdb,sorphi/rocksdb,caijieming-baidu/rocksdb,JohnPJenkins/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,vashstorm/rocksdb,vashstorm/rocksdb,Applied-Duality/rocksdb,JackLian/rocksdb,lgscofield/rocksdb,ryneli/rocksdb,Andymic/rocksdb,vmx/rocksdb,IMCG/RcoksDB,mbarbon/rocksdb,ryneli/rocksdb,biddyweb/rocksdb,IMCG/RcoksDB,sorphi/rocksdb,wskplho/rocksdb,RyanTech/rocksdb,NickCis/rocksdb,JackLian/rocksdb,amyvmiwei/rocksdb,NickCis/rocksdb,lgscofield/rocksdb