Fix NULL pointer dereference with NULL content in legacy nodes in title getting (#15558)

Author: nielsdos

ext/dom/html_document.c

@@ -1467,7 +1467,7 @@ static zend_string *dom_get_child_text_content(const xmlNode *node)
 
 	const xmlNode *text = node->children;
 	while (text != NULL) {
-		if (text->type == XML_TEXT_NODE || text->type == XML_CDATA_SECTION_NODE) {
+		if ((text->type == XML_TEXT_NODE || text->type == XML_CDATA_SECTION_NODE) && text->content != NULL) {
 			smart_str_appends(&content, (const char *) text->content);
 		}
 		text = text->next;

ext/dom/tests/modern/common/Document_title_getter.phpt

@@ -43,6 +43,10 @@ var_dump($dom->title);
 $dom = Dom\XMLDocument::createFromString("<root xmlns=\"http://www.w3.org/1999/xhtml\"><title>title\nhere</title></root>");
 var_dump($dom->title);
 
+$dom = Dom\XMLDocument::createFromString("<root xmlns=\"http://www.w3.org/1999/xhtml\"><title/></root>");
+$dom->getElementsByTagName('title')[0]->appendChild($dom->importLegacyNode(new DOMText));
+var_dump($dom->title);
+
 echo "=== SVG namespaced root ===\n";
 
 $dom = Dom\XMLDocument::createFromString("<root xmlns=\"http://www.w3.org/1999/xhtml\"><title>title</title></root>");
@@ -72,6 +76,7 @@ string(0) ""
 string(2) "xz"
 string(2) "yw"
 string(10) "title here"
+string(0) ""
 === SVG namespaced root ===
 string(5) "title"
 string(5) "title"