* calendar/todo-mode.el: Fix wrong-type-argument error when

* calendar/todo-mode.el: Fix wrong-type-argument error when
marking multiple consecutive items.
(todo-toggle-mark-item): Don't try to mark the empty lines at the
end of the todo and done items sections.  Note in doc string that
items marked by passing a numeric prefix argument can include the
last todo and first done items.
(todo-mark-category): Don't try to mark the empty line between the
todo and done items sections.