Fix compile warning in db_stress
Fix compile warning in db_stress

Summary:
Fix compile warning in db_stress

Test Plan:
make db_stress

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