fixed deficient help-delay

When I did eval of the following code, the error was shown, which is `(wrong-type-argument arrayp nil)`.

```lisp
(popup-cascade-menu '(Foo (Bar Baz) (Hoge Fuga))
                    :help-delay 1.5)
```

The cause is `lookup-key` that is called with nil by the timeout of `popup-menu-read-key-sequence`.

In addition, the menu item help was not shown because the help-delay slot is deficient when do `popup-cascade-menu` to the sublist of the menu.

Best regards.