db_bench: explicitly clear buffer in compress benchmark
db_bench: explicitly clear buffer in compress benchmark

Summary: It is reported that in compress benchmark in db_bench, zlib will cause an OOM. The suggestd fix was to clear the buffer.

Test Plan: Build and run compress benchmark.

Reviewers: IslamAbdelRahman, yhchiang, rven, andrewkr, kradhakrishnan, anthony

Reviewed By: anthony

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D52857

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