bug#17275: emacs-24 crash in dired

[Andy Moreton]

Running emacs 24 on win7 64bit built from r116968 with mingw32 gcc 4.7.2.

From "emacs -Q"
1) Open a dired buffer:
   C-x C-f ~/.emacs.d RET

2) Switch to external 'ls' program
  (setq insert-directory-program "C:/cygwin/bin/ls.exe")
  (setq ls-lisp-use-insert-directory-program t)
  (setq dired-listing-switches "-al --group-directories-first")

3) Type 'g' in the dired buffer (bound to revert-buffer)
   Emacs shows the abort dialog.
   I have not managed to get gdb to emit a usable backtrace.

This is a recent regression (within the last week) as I build emacs from 
source daily. This bug does not manifest in current emacs trunk builds.