Fix segmentation fault caused by #1961
Fix segmentation fault caused by #1961

Summary:
Fixes #1961 which causes a segfault when filter_policy is nullptr and both
pin_l0_filter_and_index_blocks_in_cache/cache_index_and_filter_blocks
are set.
Closes https://github.com/facebook/rocksdb/pull/2029

Differential Revision: D4764862

Pulled By: maysamyabandeh

fbshipit-source-id: 05bd695

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