Propagate :safe properties when autoloading defcustoms

* lisp/emacs-lisp/autoload.el (make-autoload): Propagate the :safe
property to the loaddefs file (bug#28104).

etc/NEWS

@@ -2914,6 +2914,9 @@ The former is now declared obsolete.
 
 * Lisp Changes in Emacs 28.1
 
+---
+*** :safe settings in 'defcustom' are now propagated to the loaddefs files.
+
 +++
 ** New function 'syntax-class-to-char'.
 This does almost the opposite of 'string-to-syntax' -- it returns the

lisp/emacs-lisp/autoload.el

@@ -250,7 +250,10 @@ expression, in which case we want to handle forms differently."
 	   (custom-autoload ',varname ,file
                             ,(condition-case nil
                                  (null (plist-get props :set))
-                               (error nil))))))
+                               (error nil)))
+           ;; Propagate the :safe property to the loaddefs file.
+           ,@(when-let ((safe (plist-get props :safe)))
+               `((put ',varname 'safe-local-variable ,safe))))))
 
      ((eq car 'defgroup)
       ;; In Emacs this is normally handled separately by cus-dep.el, but for