blockchain_db: initialize m_open in base class ctor

It's cleaner this way, since it's a base class field

Coverity 136568

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -1074,13 +1074,12 @@ BlockchainLMDB::~BlockchainLMDB()
     close();
 }
 
-BlockchainLMDB::BlockchainLMDB(bool batch_transactions)
+BlockchainLMDB::BlockchainLMDB(bool batch_transactions): BlockchainDB()
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
   // initialize folder to something "safe" just in case
   // someone accidentally misuses this class...
   m_folder = "thishsouldnotexistbecauseitisgibberish";
-  m_open = false;
 
   m_batch_transactions = batch_transactions;
   m_write_txn = nullptr;