Fix GH-16149: Null pointer dereference in DOMElement->getAttributeNames()

A namespace without a prefix is by definition always the "xmlns"
namespace.

Closes GH-16155.

NEWS

@@ -14,6 +14,8 @@ PHP                                                                        NEWS
 - DOM:
   . Fixed bug GH-16039 (Segmentation fault (access null pointer) in
     ext/dom/parentnode/tree.c). (nielsdos)
+  . Fixed bug GH-16149 (Null pointer dereference in
+    DOMElement->getAttributeNames()). (nielsdos)
 
 - JSON:
   . Fixed bug GH-15168 (stack overflow in json_encode()). (nielsdos)

ext/dom/element.c

@@ -339,7 +339,11 @@ PHP_METHOD(DOMElement, getAttributeNames)
 
 	for (xmlNsPtr nsptr = nodep->nsDef; nsptr; nsptr = nsptr->next) {
 		const char *prefix = (const char *) nsptr->prefix;
-		ZVAL_STR(&tmp, dom_node_concatenated_name_helper(strlen(prefix), prefix, strlen("xmlns"), (const char *) "xmlns"));
+		if (prefix == NULL) {
+			ZVAL_STRING(&tmp, "xmlns");
+		} else {
+			ZVAL_STR(&tmp, dom_node_concatenated_name_helper(strlen(prefix), prefix, strlen("xmlns"), (const char *) "xmlns"));
+		}
 		zend_hash_next_index_insert(ht, &tmp);
 	}
 

ext/dom/tests/gh16149.phpt

@@ -0,0 +1,14 @@
+--TEST--
+GH-16149 (Null pointer dereference in DOMElement->getAttributeNames())
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+$element = new DOMElement("b", null, "a");
+var_dump($element->getAttributeNames());
+?>
+--EXPECT--
+array(1) {
+  [0]=>
+  string(5) "xmlns"
+}