mod_s2s: Bail if connection is destroyed after attempting to open stream Fixes "attempt to compare number with nil" because `session.version` has been cleared by s2smanager.destroy_session. This can happen with the server_epoll setting opportunistic_writes enabled, which means that it can notice that the connection failed at this point, after which it triggers the whole chain of events that leads to session destruction and "cleaning" most of the session fields.