Fix GH-16808: Segmentation fault in RecursiveIteratorIterator->current() with a xml element input

When the current data is invalid, NULL must be returned. At least that's
how the check in SPL works and how other extensions do this as well.
If we don't do this, an UNDEF value gets propagated to a return value
(misprinted as null); leading to issues.

Author: ndossche

ext/simplexml/simplexml.c

@@ -2539,7 +2539,11 @@ static zval *php_sxe_iterator_current_data(zend_object_iterator *iter) /* {{{ */
 {
 	php_sxe_iterator *iterator = (php_sxe_iterator *)iter;
 
-	return &iterator->sxe->iter.data;
+	zval *data = &iterator->sxe->iter.data;
+	if (Z_ISUNDEF_P(data)) {
+		return NULL;
+	}
+	return data;
 }
 /* }}} */
 

ext/simplexml/tests/gh16808.phpt

@@ -0,0 +1,12 @@
+--TEST--
+GH-16808 (Segmentation fault in RecursiveIteratorIterator->current() with a xml element input)
+--EXTENSIONS--
+simplexml
+--FILE--
+<?php
+$sxe = new SimpleXMLElement("<root />");
+$test = new RecursiveIteratorIterator($sxe);
+var_dump($test->current());
+?>
+--EXPECT--
+NULL