merged ut_metadata crash fix from RC_0_16

ChangeLog

@@ -9,6 +9,7 @@
 	* fix uTP edge case where udp socket buffer fills up
 	* fix nagle implementation in uTP
 
+	* fixed rare crash in ut_metadata extension
 	* fixed files checking issue
 	* added missing pop_alerts() to python bindings
 	* fixed typos in configure script, inversing some feature-enable/disable flags

src/ut_metadata.cpp

@@ -405,6 +405,8 @@ namespace libtorrent { namespace
 
 		void maybe_send_request()
 		{
+			if (m_pc.is_disconnecting()) return;
+
 			// if we don't have any metadata, and this peer
 			// supports the request metadata extension
 			// and we aren't currently waiting for a request