Fix memory leak on blob db open
Fix memory leak on blob db open

Summary:
Fixes #2820
Closes https://github.com/facebook/rocksdb/pull/2826

Differential Revision: D5757527

Pulled By: yiwu-arbug

fbshipit-source-id: f495b63700495aeaade30a1da5e3675848f3d72f

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