Fix regression failure in PrefixTest.PrefixValid
Fix regression failure in PrefixTest.PrefixValid

Summary: Use IterKey to store prefix_start_ so that it doesn't get freed

Test Plan: PrefixTest.PrefixValid

Reviewers: anthony, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D50289

Vaisman/rocksdb,tsheasha/rocksdb,vmx/rocksdb,hobinyoon/rocksdb,wenduo/rocksdb,SunguckLee/RocksDB,tsheasha/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,hobinyoon/rocksdb,ryneli/rocksdb,Andymic/rocksdb,hobinyoon/rocksdb,jalexanderqed/rocksdb,OverlordQ/rocksdb,facebook/rocksdb,hobinyoon/rocksdb,bbiao/rocksdb,bbiao/rocksdb,tsheasha/rocksdb,Vaisman/rocksdb,Andymic/rocksdb,bbiao/rocksdb,tsheasha/rocksdb,norton/rocksdb,OverlordQ/rocksdb,jalexanderqed/rocksdb,OverlordQ/rocksdb,SunguckLee/RocksDB,norton/rocksdb,jalexanderqed/rocksdb,bbiao/rocksdb,norton/rocksdb,tsheasha/rocksdb,facebook/rocksdb,tsheasha/rocksdb,facebook/rocksdb,wenduo/rocksdb,tsheasha/rocksdb,wenduo/rocksdb,Vaisman/rocksdb,hobinyoon/rocksdb,flabby/rocksdb,vmx/rocksdb,norton/rocksdb,wat-ze-hex/rocksdb,jalexanderqed/rocksdb,vmx/rocksdb,ryneli/rocksdb,SunguckLee/RocksDB,norton/rocksdb,facebook/rocksdb,facebook/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,OverlordQ/rocksdb,flabby/rocksdb,facebook/rocksdb,wenduo/rocksdb,hobinyoon/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,ryneli/rocksdb,flabby/rocksdb,Vaisman/rocksdb,wenduo/rocksdb,hobinyoon/rocksdb,norton/rocksdb,hobinyoon/rocksdb,Andymic/rocksdb,facebook/rocksdb,vmx/rocksdb,wenduo/rocksdb,SunguckLee/RocksDB,ryneli/rocksdb,norton/rocksdb,vmx/rocksdb,ryneli/rocksdb,jalexanderqed/rocksdb,wenduo/rocksdb,SunguckLee/RocksDB,jalexanderqed/rocksdb,bbiao/rocksdb,facebook/rocksdb,ryneli/rocksdb,Vaisman/rocksdb,flabby/rocksdb,flabby/rocksdb,bbiao/rocksdb,wenduo/rocksdb,bbiao/rocksdb,OverlordQ/rocksdb,flabby/rocksdb,bbiao/rocksdb,vmx/rocksdb,tsheasha/rocksdb,Andymic/rocksdb,Andymic/rocksdb,Vaisman/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,jalexanderqed/rocksdb,OverlordQ/rocksdb,ryneli/rocksdb,SunguckLee/RocksDB,norton/rocksdb,wat-ze-hex/rocksdb,flabby/rocksdb,OverlordQ/rocksdb,Vaisman/rocksdb,wat-ze-hex/rocksdb