NgaiShunChan

In file gorocksdb.c, an empty implementation of function gorocksdb_filterpolicy_delete_filter and gorocksdb_mergeoperator_delete_value preventing default function free to be called. See [rocksdb/blob/master/db/c.cc](https://github.com/facebook/rocksdb/blob/master/db/c.cc) Which may lead to memory leak.