Fix todo-mode item date editing bugs

Fix todo-mode item date editing bugs

* lisp/calendar/todo-mode.el (todo-edit-item--header): Prevent out of
range error by making sure the value of the numerical month date
component cannot be nil.  Prevent wrong type error on trying to edit
day number in February by making sure numerical instead of string
value of the year component is passed to todo-read-date.
(todo-read-date): When using the numerical month date component make
sure to use `*' for an arbitrary month instead of its numerical value.