Update mod/messages/start.php

We had an Elgg user named Chris Read with username 'read'. Once he registered, people's messages stopped working because hitting a message in your inbox was a url like: [site_name]/messages/read/459 - and the message code, supporting the old URL format, looked up the parameter right after messages and did a lookup on that word. So, since it got a user, redirected to his inbox. Yipes! So I put in some code checking that the parameter really is your username, so it would work for Chris, but not for anyone else. It works fine now.