back port check to not destruct session_impl from its own thread

include/libtorrent/aux_/session_impl.hpp

@@ -271,6 +271,14 @@ namespace libtorrent
 #if defined BOOST_HAS_PTHREADS
 				if (m_network_thread == 0) return true;
 				return m_network_thread == pthread_self();
+#endif
+				return true;
+			}
+			bool is_not_network_thread() const
+			{
+#if defined BOOST_HAS_PTHREADS
+				if (m_network_thread == 0) return true;
+				return m_network_thread != pthread_self();
 #endif
 				return true;
 			}

src/session_impl.cpp

@@ -5940,6 +5940,8 @@ retry:
 
 	session_impl::~session_impl()
 	{
+		TORRENT_ASSERT(is_not_network_thread());
+
 		m_io_service.post(boost::bind(&session_impl::abort, this));
 
 		// we need to wait for the disk-io thread to