Error in og_panels_node_data()

Prevent PHP notices and make sure array() is returned is $tab_num does not exist.

   if ($tab_num) {
-    return $data[$nid][$tab_num];
+    return isset($data[$nid][$tab_num]) ? $data[$nid][$tab_num] : array();
   }
-  elseif (og_panels_node_data) {
+  elseif (isset($data[$nid])) {
     return $data[$nid];
   }