don't restart subscriptions update timer if no subscription presented

AddressBook.cpp

@@ -355,11 +355,14 @@ namespace client
 	void AddressBook::DownloadComplete (bool success)
 	{
 		m_IsDownloading = false;
+		if (m_SubscriptionsUpdateTimer)
+		{
 			m_SubscriptionsUpdateTimer->expires_from_now (boost::posix_time::minutes(
 				success ? CONTINIOUS_SUBSCRIPTION_UPDATE_TIMEOUT : CONTINIOUS_SUBSCRIPTION_RETRY_TIMEOUT));
 			m_SubscriptionsUpdateTimer->async_wait (std::bind (&AddressBook::HandleSubscriptionsUpdateTimer,
 				this, std::placeholders::_1));
 		}
+	}
 
 	void AddressBook::StartSubscriptions ()
 	{