fix valgrind failure in backupable_db_test
fix valgrind failure in backupable_db_test

Summary: fix memory leak in test code

Test Plan: ran test

Reviewers: rven, igor, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D52617

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