blockchain: initialize m_hardfork to NULL

It can now be set by some other code, and is thus tested

src/cryptonote_core/blockchain.cpp

@@ -100,7 +100,7 @@ static const uint64_t testnet_hard_fork_version_1_till = 624633;
 
 //------------------------------------------------------------------
 Blockchain::Blockchain(tx_memory_pool& tx_pool) :
-  m_db(), m_tx_pool(tx_pool), m_hardfork(), m_timestamps_and_difficulties_height(0), m_current_block_cumul_sz_limit(0), m_is_in_checkpoint_zone(false),
+  m_db(), m_tx_pool(tx_pool), m_hardfork(NULL), m_timestamps_and_difficulties_height(0), m_current_block_cumul_sz_limit(0), m_is_in_checkpoint_zone(false),
   m_is_blockchain_storing(false), m_enforce_dns_checkpoints(false), m_max_prepare_blocks_threads(4), m_db_blocks_per_sync(1), m_db_sync_mode(db_async), m_fast_sync(true), m_sync_counter(0)
 {
   LOG_PRINT_L3("Blockchain::" << __func__);