bug#14688: 24.3; todo-show errors: wrong-type-argument and file-error

[Leo Liu]

Make sure nothing is in ~/.emacs.d/todo

1. start emacs
2. todo-show and answer its queries and save the buffer
3. go to ~/.emacs.d/todo and delete files there
4. M-x todo-show

you will get a couple of errors such as:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  expand-file-name(nil)
  find-file-noselect(nil)
  todo-read-category("Enter a new category name: " add nil)
  todo-add-category(nil "")
  byte-code
  todo-show(nil)
  call-interactively(todo-show record nil)
  command-execute(todo-show record)
  execute-extended-command(nil "todo-show")
  call-interactively(execute-extended-command nil nil)
  command-execute(execute-extended-command)

Debugger entered--Lisp error: (file-error "Removing old name" "no such file or 
directory" "/home/leo/.emacs.d/todo/TODO.todo")
  delete-file("/home/leo/.emacs.d/todo/TODO.todo")
  byte-code
  byte-code
  todo-show(nil)
  call-interactively(todo-show record nil)
  command-execute(todo-show record)
  execute-extended-command(nil "todo-show")
  call-interactively(execute-extended-command nil nil)
  command-execute(execute-extended-command)