Fixed a couple of issues with EVENT_TEXT_RSS

1. The event was getting $p_string rather than $t_string which is already processed by previous calls.
2. Trigger the even before converting the text to be RSS friendly.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@4721 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9