fix issue where shutting down the session immediately and asynchronously starting the DHT would cause a shutdown hang (#1203)

src/session_impl.cpp

@@ -5805,6 +5805,8 @@ retry:
 		// postpone starting the DHT if we're still resolving the DHT router
 		if (m_outstanding_router_lookups > 0) return;
 
+		if (m_abort) return;
+
 		m_dht = boost::make_shared<dht::dht_tracker>(static_cast<dht_observer*>(this)
 			, boost::ref(m_udp_socket), boost::cref(m_dht_settings)
 			, boost::ref(m_stats_counters)

test/test_session.cpp

@@ -256,5 +256,11 @@ TORRENT_TEST(save_restore_state_load_filter)
 		});
 }
 
+TORRENT_TEST(session_shutdown)
+{
+	lt::settings_pack pack;
+	lt::session ses(pack);
+}
+
 #endif