Ensure $Discussion is passed by reference

`$Discussion ` is not passed by reference all the way through the calls beginning from the `getWhere()` method; the discussion data cannot be modified from the `SetCalculatedFields` event when the `getWhere()` method is used when it should be able to.

applications/vanilla/models/class.discussionmodel.php

@@ -736,7 +736,7 @@ public function calculate(&$Discussion) {
             $Discussion->LastDate = $Discussion->DateInserted;
         }
 
-        $this->EventArguments['Discussion'] = $Discussion;
+        $this->EventArguments['Discussion'] = &$Discussion;
         $this->fireEvent('SetCalculatedFields');
     }