Fix nullptr dereference in adaptive_table
Fix nullptr dereference in adaptive_table

Summary:
@dulmarod Ran infer on RocksDB and found that we dereference nullptr in adaptive_table
https://fb.facebook.com/groups/rocksdb/permalink/1046374415411173/

Test Plan: make check -j64

Reviewers: sdong, yhchiang, andrewkr

Reviewed By: andrewkr

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D56973

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