vpn: ensure valid specific object when none was given at activation

If the caller passed "/" to indicate that NM should choose the default
active connection as the parent of the VPN, make sure we pass that
active connection's object path to the VPN for its specific object
path instead of leaving it "/".

src/nm-manager.c

@@ -1992,7 +1992,7 @@ nm_manager_activate_connection (NMManager *manager,
 		vpn_connection = nm_vpn_manager_activate_connection (priv->vpn_manager,
 		                                                     connection,
 		                                                     device,
-		                                                     specific_object,
+		                                                     nm_act_request_get_active_connection_path (parent_req),
 		                                                     TRUE,
 		                                                     sender_uid,
 		                                                     error);