The chat_get_latest_message function does not discriminate if chat messages have the same timestamp

This bug affects all moodle version, i think.

The bug is in the chat lib (mod/chat/lib.php), in the function chat_get_latest_message($chatid, $groupid=0)

If the chat message have the same timestamp, it could get wrong last chat message,
this is cause by this lines (line 440)

if (!$rs = $db->SelectLimit("SELECT *
FROM {$CFG->prefix}chat_messages
WHERE chatid = '$chatid' $groupselect
ORDER BY timestamp DESC", 1))

it's could be fix by change to this code:

if (!$rs = $db->SelectLimit("SELECT *
FROM {$CFG->prefix}chat_messages
WHERE chatid = '$chatid' $groupselect
ORDER BY timestamp DESC, id DESC", 1))

or with this query that not use any limit or sort

SELECT * FROM mdl_chat_messages WHERE chatid = <CHAT_ID> and timestamp=(SELECT MAX(timestamp) FROM mdl_chat_messages WHERE chatid = <CHAT_ID>) and id=(SELECT MAX(id) from mdl_chat_messages WHERE chatid = <CHAT_ID>);