bug#9814: 23.3; unguarded setq of default-directory in ido.el

[Dave Abrahams <dave@boostpro.com>]

I'm sorry that I don't have a reproducer at the moment.  I was having
some failures during startup that would leave my Emacs unable to change
buffers with `M-x switch-to-buffer', which was remapped to
ido-switch-buffer, because it would attempt to make a Tramp connection
to some nonexistent host.  I could get around this by `M-: (setq
default-directory "~")', so I went hunting in ido.el for mentions of
default-directory and found 2 instances of (setq default-directory...)
that were not apparently surrounded by a let.  This seems like a likely
cause of the problem.

In GNU Emacs 23.3.1 (x86_64-apple-darwin10.8.0, Carbon Version 1.6.0 AppKit 1038.36)
 of 2011-09-12 on pluto.luannocracy.com
Windowing system distributor `Apple Inc.', version 10.6.8
configured using `configure  '--with-mac' '--enable-mac-app' '--prefix=/usr/local/stow/emacs-23.3' 'CC=gcc -arch x86_64''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Fundamental

Minor modes in effect:
  highlight-changes-visible-mode: t
  diff-auto-refine-mode: t
  recentf-mode: t
  pretty-control-l-mode: t
  Info-breadcrumbs-in-mode-line-mode: t
  workgroups-mode: t
  shell-dirtrack-mode: t
  me-minor-mode: t
  show-paren-mode: t
  ido-everywhere: t
  global-auto-revert-mode: t
  display-time-mode: t
  delete-selection-mode: t
  tooltip-mode: t
  mac-mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
C-p C-p C-p C-p C-p C-p C-p C-p C-p C-n C-n C-p C-n 
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n