MDL-64240 mod_forum: Clean forum post text before storing

Previously only edited messages were cleaned, so longer strings could
be stored in the database, which would never be displayed.
This update takes into account trusted text before cleaning (as editing does).

mod/forum/post.php

@@ -696,6 +696,9 @@ if ($mformpost->is_cancelled()) {
     // WARNING: the $fromform->message array has been overwritten, do not use it anymore!
     $fromform->messagetrust  = trusttext_trusted($modcontext);
 
+    // Clean message text.
+    $fromform = trusttext_pre_edit($fromform, 'message', $modcontext);
+
     if ($fromform->edit) {           // Updating a post.
         unset($fromform->groupid);
         $fromform->id = $fromform->edit;