ext/xsl: handle xsltNewTransformContext allocation failure

xsltNewTransformContext can return NULL on libxslt-internal allocation
failure. The next line dereferences ctxt unconditionally to set
ctxt->_private, segfaulting on OOM. Bail through the existing out:
label, which already handles secPrefs/intern->doc cleanup.

xsltFreeTransformContext(NULL) is a documented no-op in libxslt.

Author: iliaal

ext/xsl/xsltprocessor.c

@@ -350,6 +350,9 @@ static xmlDocPtr php_xsl_apply_stylesheet(zval *id, xsl_object *intern, xsltStyl
 	php_libxml_increment_doc_ref(intern->doc, doc);
 
 	ctxt = xsltNewTransformContext(style, doc);
+	if (UNEXPECTED(ctxt == NULL)) {
+		goto out;
+	}
 	ctxt->_private = (void *) intern;
 
 	if (intern->parameter) {