net.xmppcomponent_listener: Call session:on_destroy() on session disconnect.

net/xmppcomponent_listener.lua

@@ -203,6 +203,7 @@ function component_listener.ondisconnect(conn, err)
 	local session = sessions[conn];
 	if session then
 		(session.log or log)("info", "component disconnected: %s (%s)", tostring(session.host), tostring(err));
+		if session.on_destroy then session:on_destroy(err); end
 		if hosts[session.host] then
 			log("debug", "Deregistering component");
 			cm_deregister_component(session.host);