Posts, Post Types: Ensure is_page_template() can only return true when viewing a singular post query.

Props natereist, dlh
Fixes #39211


git-svn-id: https://develop.svn.wordpress.org/trunk@39599 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post-template.php

@@ -1631,6 +1631,10 @@ function get_the_password_form( $post = 0 ) {
  * @return bool True on success, false on failure.
  */
 function is_page_template( $template = '' ) {
+	if ( ! is_singular() ) {
+		return false;
+	}
+
 	$page_template = get_page_template_slug( get_queried_object_id() );
 
 	if ( empty( $template ) )