blockchain: call deinit in dtor

This ensures the io service that runs in another thread cannot
access data after it's deleted

src/cryptonote_core/blockchain.cpp

@@ -171,6 +171,11 @@ Blockchain::Blockchain(tx_memory_pool& tx_pool) :
   LOG_PRINT_L3("Blockchain::" << __func__);
 }
 //------------------------------------------------------------------
+Blockchain::~Blockchain()
+{
+  deinit();
+}
+//------------------------------------------------------------------
 bool Blockchain::have_tx(const crypto::hash &id) const
 {
   LOG_PRINT_L3("Blockchain::" << __func__);
@@ -550,16 +555,14 @@ bool Blockchain::deinit()
   // as this should be called if handling a SIGSEGV, need to check
   // if m_db is a NULL pointer (and thus may have caused the illegal
   // memory operation), otherwise we may cause a loop.
-  if (m_db == NULL)
-  {
-    throw DB_ERROR("The db pointer is null in Blockchain, the blockchain may be corrupt!");
-  }
-
   try
+  {
+    if (m_db)
     {
       m_db->close();
       MTRACE("Local blockchain read/write activity stopped successfully");
     }
+  }
   catch (const std::exception& e)
   {
     LOG_ERROR(std::string("Error closing blockchain db: ") + e.what());

src/cryptonote_core/blockchain.h

@@ -120,6 +120,11 @@ namespace cryptonote
      */
     Blockchain(tx_memory_pool& tx_pool);
 
+    /**
+     * @brief Blockchain destructor
+     */
+    ~Blockchain();
+
     /**
      * @brief Initialize the Blockchain state
      *