Fix bug that caused really-long sub-board load times.

I'm not sure how long this has been like this, but the last_msg
of a sub-board can be a super high number (e.g. in the billions), so using
that value as the highest message offset would cause tons of message index
read failures and just cause this loop to interate unneccessarily through
tons of non-existent messages. Also, this code is utter crap.

web/lib/msgslib.ssjs

@@ -52,7 +52,8 @@ function get_message_offsets(type)
 //	var hdr;
 
 //	for(last_offset=0; (idx=msgbase.get_msg_index(true,last_offset)) != null;last_offset++) {
-	for (var last_offset = 0; last_offset <= msgbase.last_msg; last_offset++) {
+	var total_msgs = msgbase.total_msgs;
+	for (var last_offset = 0; last_offset < msgbase.total_msgs; last_offset++) {
 		var idx = msgbase.get_msg_index(true, last_offset);
 		if (typeof idx === 'undefined' || idx === null) continue;
 		if(idx.attr&MSG_DELETE)