Fix use-after-free in c_tset
Fix use-after-free in c_tset

Summary:
Fix asan error introduce by #2823
Closes https://github.com/facebook/rocksdb/pull/2879

Differential Revision: D5828454

Pulled By: yiwu-arbug

fbshipit-source-id: 50777855667f4e7b634279a654c3bfa01a1ac729

Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,bbiao/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,bbiao/rocksdb,Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,bbiao/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,bbiao/rocksdb,Andymic/rocksdb,bbiao/rocksdb,Andymic/rocksdb,bbiao/rocksdb,bbiao/rocksdb