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/berkeleydb/db_bdb.cpp

@@ -770,13 +770,13 @@ BlockchainBDB::~BlockchainBDB()
 }
 
 BlockchainBDB::BlockchainBDB(bool batch_transactions) :
+        BlockchainDB(),
         m_buffer(DB_BUFFER_COUNT, DB_BUFFER_LENGTH)
 {
     LOG_PRINT_L3("BlockchainBDB::" << __func__);
     // initialize folder to something "safe" just in case
     // someone accidentally misuses this class...
     m_folder = "thishsouldnotexistbecauseitisgibberish";
-    m_open = false;
     m_run_checkpoint = 0;
     m_batch_transactions = batch_transactions;
     m_write_txn = nullptr;

src/blockchain_db/blockchain_db.h

@@ -537,6 +537,11 @@ protected:
 
 public:
 
+  /**
+   * @brief An empty constructor.
+   */
+  BlockchainDB(): m_open(false) { }
+
   /**
    * @brief An empty destructor.
    */

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;