Moving $_GET['topicId'] code in to existing conditional

include/client/open.inc.php

@@ -9,12 +9,13 @@ if($thisclient && $thisclient->isValid()) {
 
 $info=($_POST && $errors)?Format::htmlchars($_POST):$info;
 
-if(array_key_exists('topicId',$_GET) && preg_match('/^\d+$/',$_GET['topicId']))
-    $info['topicId'] = intval($_GET['topicId']);
-
 $form = null;
-if (!$info['topicId'])
-    $info['topicId'] = $cfg->getDefaultTopicId();
+if (!$info['topicId']) {
+    if (array_key_exists('topicId',$_GET) && preg_match('/^\d+$/',$_GET['topicId']) && Topic::lookup($_GET['topicId']))
+        $info['topicId'] = intval($_GET['topicId']);
+    else
+        $info['topicId'] = $cfg->getDefaultTopicId();
+}
 
 if ($info['topicId'] && ($topic=Topic::lookup($info['topicId']))) {
     $form = $topic->getForm();