Description:
------------
isset on nonexisting nodes return unexpected results

Reproduce code:
---------------
$a = simplexml_load_string("
        <r>
                <p>Test</p>
                <o d='h'>
                          <xx rr='info' />
                          <yy rr='data' />
                </o>
        </r>
");

var_dump(isset($a->o->zz));
var_dump(isset($a->o->text));

Expected result:
----------------
bool(false)
bool(false)

Actual result:
--------------
bool(false)
bool(true)