Fix GH-21688: SEGV in C14N on empty HTMLDocument.

close GH-21693

Author: devnexen

NEWS

@@ -15,6 +15,8 @@ PHP                                                                        NEWS
 - DOM:
   . Fixed bug GH-21566 (Dom\XMLDocument::C14N() emits duplicate xmlns
     declarations after setAttributeNS()). (David Carlier)
+  . Fixed bug GH-21688 (segmentation fault on empty HTMLDocument).
+    (David Carlier)
 
 - Iconv:
   . Fixed bug GH-17399 (iconv memory leak on bailout). (iliaal)

ext/dom/node.c

@@ -2263,7 +2263,11 @@ static void dom_canonicalization(INTERNAL_FUNCTION_PARAMETERS, int mode) /* {{{
 		}
 
 		zend_hash_init(&links, 0, NULL, NULL, false);
-		dom_relink_ns_decls(&links, xmlDocGetRootElement(docp));
+		xmlNodePtr root_element = xmlDocGetRootElement(docp);
+
+		if (root_element) {
+			dom_relink_ns_decls(&links, root_element);
+		}
 	} else if (!docp) {
 		/* Note: not triggerable with modern DOM */
 		zend_throw_error(NULL, "Node must be associated with a document");

ext/dom/tests/modern/html/gh21688.phpt

@@ -0,0 +1,13 @@
+--TEST--
+GH-21688 (SEGV in C14N on empty HTMLDocument)
+--CREDITS--
+YuanchengJiang
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+$dom = Dom\HTMLDocument::createEmpty();
+var_dump($dom->C14N());
+?>
+--EXPECT--
+string(0) ""