Dynamic widget is searching for item key in item prototypes

Steps to reproduce:

1. Create host H1 with discovery rule with item prototype with key "x"
2. Create other host H2 with item I2 with key "x"
3. On dashboard create Simple graph widget with item I2.
4. Mark widget as Dynamic. Save dashboard.
5. Select H1 as source for dynamic widgets.
   Result:

   Undefined offset: 0 [zabbix.php:21 ? require_once() ? ZBase->run() ? ZBase->processRequest() ? CController->run() ? CControllerWidgetGraphView->doAction() in app\controllers\CControllerWidgetGraphView.php:151]

   Expected:
   Message: "No permissions to referred object or it does not exist!"

This happens, because graph widget uses function get_same_item_for_host(), that searches for item prototypes, as well as items.