Fix auto-save with user-emacs-directory

* lisp/startup.el (auto-save-list-file-prefix): Delay initialization,
since the value depends on user-emacs-directory (Bug#37354).

lisp/startup.el

@@ -406,6 +406,7 @@ if you have not already set `auto-save-list-file-name' yourself.
 Directories in the prefix will be created if necessary.
 Set this to nil if you want to prevent `auto-save-list-file-name'
 from being initialized."
+  :initialize #'custom-initialize-delay
   :type '(choice (const :tag "Don't record a session's auto save list" nil)
 		 string)
   :group 'auto-save)