mod_smacks: Prevent traceback on attempting to resume destroyed session

This shouldn't be possible, but seems to happen anyway after net.server
somehow calling the close handler twice, which also should not happen.

plugins/mod_smacks.lua

@@ -587,6 +587,11 @@ function do_resume(session, stanza)
 
 	local id = stanza.attr.previd;
 	local original_session = session_registry[registry_key(session, id)];
+	if original_session.destroyed then
+		original_session.log("error", "Tried to resume a destroyed session. This should not happen! %s", debug.traceback());
+		session_registry[registry_key(session, id)] = nil;
+		original_session = nil;
+	end
 	if not original_session then
 		local old_session = old_session_registry:get(session.username, id);
 		if old_session then