db_bench: Missing cleanup of db_ member in DBWithColumnFamilies::DeleteDBs()

[udi-speedb]

When FLAGS_optimistic_transaction_db is true (Non-ROCKSDB_LITE), the following method sets opt_txn_db to nullptr, but fails to also set db_ to nullptr. db_ should be set to nullptr in all paths.

  void DeleteDBs() {
    std::for_each(cfh.begin(), cfh.end(),
                  [](ColumnFamilyHandle* cfhi) { delete cfhi; });
    cfh.clear();
#ifndef ROCKSDB_LITE
    if (opt_txn_db) {
      delete opt_txn_db;
      opt_txn_db = nullptr;
    } else {
      delete db;
      db = nullptr;
    }
#else
    delete db;
    db = nullptr;
#endif  // ROCKSDB_LITE
  }

[udi-speedb]

Looking at the code now I don't really see a scenario to actually cause any bugs. Will be reopened if any issue is found.