recentf: update menu failed

[Glenn Morris]

Since the recent (ahem) changes to recentf.el, I get this message
many times when using VM:

recentf update menu failed: Malformed menu in easy-menu: (undefined)

The error comes from easy-menu-get-map, called from
easy-menu-add-item, called from easy-menu-change, called from
recentf-update-menu.

In the easy-menu-get-map call, MAP is nil, and PATH is ("files").

Inside easy-menu-get-map:
In the let* initialization, key is set to [menu-bar files].

The VM mode menu map is defined (vm-menu.el) to contain entries such
as this:

        ;; block the global menubar entries in the map so that VM
        ;; can take over the menubar if necessary.
           (tools . undefined)
           (files . undefined)

[What VM does is to completely take over the menu-bar with
VM-specific menu entries that replace the standard Emacs ones].

The net result is that a lookup-key for [menu-bar files] returns
'undefined, causing easy-menu-get-map to throw an error because it's
not a keymap.