Fix for unit test ipc.asynchronous (nanocurrency#3964)

It can fail due to the callback trying to access a destroyed node.
The real problem is likely larger than this little fix and related
to the chaotic way the node shuts down. Leaving the larger fix for
another time, priorities...

nano/node/ipc/ipc_server.cpp

@@ -511,23 +511,30 @@ class socket_transport : public nano::ipc::transport
 		// Prepare the next session
 		auto new_session (std::make_shared<session<SOCKET_TYPE>> (server, context (), config_transport));
 
-		acceptor->async_accept (new_session->get_socket (), [this, new_session] (boost::system::error_code const & ec) {
+		std::weak_ptr<nano::node> nano_weak = server.node.shared ();
+		acceptor->async_accept (new_session->get_socket (), [this, new_session, nano_weak] (boost::system::error_code const & ec) {
+			auto node = nano_weak.lock ();
+			if (!node)
+			{
+				return;
+			}
+
 			if (!ec)
 			{
 				new_session->read_next_request ();
 			}
 			else
 			{
-				server.node.logger.always_log ("IPC: acceptor error: ", ec.message ());
+				node->logger.always_log ("IPC: acceptor error: ", ec.message ());
 			}
 
 			if (ec != boost::asio::error::operation_aborted && acceptor->is_open ())
 			{
-				this->accept ();
+				accept ();
 			}
 			else
 			{
-				server.node.logger.always_log ("IPC: shutting down");
+				node->logger.always_log ("IPC: shutting down");
 			}
 		});
 	}