mod_s2s: fix possible traceback in the case the outgoing connection f…

…ails prehand.

plugins/mod_s2s/mod_s2s.lua

@@ -56,7 +56,7 @@ local function time_and_clean(_session, now)
 		module:log("debug", "checking outgoing streams for inactivity...");
 		for _, host in pairs(hosts) do
 			for domain, session in pairs(host.s2sout) do
-				if now - session.last_send > max_inactivity then session:close(); end
+				if now - (session.last_send or 0) > max_inactivity then session:close(); end
 			end
 		end
 		last_inactive_clean = now;