Todo mode crashes when activated in an empty, file-less buffer.

Reproduced by the following recipe from Emacs -Q:
  1. Switch to a new buffer, so that it's empty, and not connected to
     a file.
  2. Run `M-x todo-mode'.

This should produce the following error message:

  todo-mode: Wrong type argument: arrayp, nil


In GNU Emacs 25.0.50.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.14.4)
 of 2014-11-19 on x240
Repository revision: f5d77aafa9d59c4afb13d744e67a0e4daf3be31a
Windowing system distributor `The X.Org Foundation', version 
11.0.11601901
System Description:	Debian GNU/Linux testing (jessie)

Configured using:
 `configure --without-makeinfo'

Configured features:
XPM JPEG TIFF GIF PNG SOUND DBUS GSETTINGS NOTIFY LIBXML2 FREETYPE XFT
ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Todo

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  line-number-mode: t
  visual-line-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Mark set
todo-mode: Wrong type argument: arrayp, nil

Load-path shadows:
None found.

Features:
(shadow sort gnus-util mail-extr emacsbug message dired format-spec
        rfc822 mml mml-sec mm-decode mm-bodies mm-encode mail-parse 
rfc2231
        mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 
ietf-drums
        mm-util help-fns mail-prsvr mail-utils todo-mode cl-loaddefs 
cl-lib
        diary-lib diary-loaddefs cal-menu easymenu calendar cal-loaddefs
        time-date tooltip eldoc electric uniquify ediff-hook vc-hooks
        lisp-float-type mwheel x-win x-dnd tool-bar dnd fontset image 
regexp-opt
        fringe tabulated-list newcomment elisp-mode lisp-mode prog-mode 
register
        page menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock
        font-lock syntax facemenu font-core frame cham georgian 
utf-8-lang
        misc-lang vietnamese tibetan thai tai-viet lao korean japanese 
hebrew
        greek romanian slovak czech european ethiopic indian cyrillic 
chinese
        case-table epa-hook jka-cmpr-hook help simple abbrev minibuffer 
nadvice
        loaddefs button faces cus-face macroexp files text-properties 
overlay
        sha1 md5 base64 format env code-pages mule custom widget
        hashtable-print-readable backquote make-network-process dbusbind
        gfilenotify dynamic-setting system-font-setting 
font-render-setting
        move-toolbar gtk x-toolkit x multi-tty emacs)

Memory information:
((conses 16 88280 6384)
 (symbols 48 19439 0)
 (miscs 40 47 110)
 (strings 32 15157 4094)
 (string-bytes 1 473901)
 (vectors 16 10700)
 (vector-slots 8 408232 3709)
 (floats 8 71 76)
 (intervals 56 196 0)
 (buffers 976 11)
 (heap 1024 33888 1101))