bug#24471: 25.1.50; Error on empty PATH component

[Achim Gratz <Stromeko@nexgo.de>]

Noam Postavsky writes:
> (Extra ";" in PATH result in "." in exec-path, which may be wrong but
> does not trigger an error.)

That's another bug, I think, due to folding POSIX and Windows
functionality.  Empty PATH elements mean "." in POSIX, but to the best
of my knowledge, they don't mean anything special on Windows, so they
should be removed or ignored.

Here's the (redacted) backtrace leading to the error I encountered.

--8<---------------cut here---------------start------------->8---
Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  directory-file-name(nil)
  mapcar(directory-file-name ("c:/WINDOWS/SYSTEM32/" "c:/WINDOWS/" "c:/WINDOWS/SYSTEM32/WBEM/" nil "c:/Local_Install/share/emacs-25.1/libexec/emacs/25.1/x86_64-w64-mingw32/"))
  eval((mapcar (quote directory-file-name) (append (parse-colon-path (getenv "PATH")) (list exec-directory))))
  custom-push-theme(theme-value exec-path user set (quote ("/usr/bin")))
  custom-theme-set-variables(user ((exec-path (quote ("/usr/bin")))))
  apply(custom-theme-set-variables user ((exec-path (quote ("/usr/bin")))))
  custom-set-variables((exec-path (quote ("/usr/bin"))))
  eval-buffer(#<buffer  *load*> nil "//server/Gratz/GNU/.emacs" nil t)  ; Reading at buffer position 3839
  load-with-code-conversion("//server/Gratz/GNU/.emacs" "//server/Gratz/GNU/.emacs" t t)
  load("~/.emacs" t t)
  [compiled]
  command-line()
  normal-top-level()

In GNU Emacs 25.1.1 (x86_64-w64-mingw32)
 of 2016-09-17 built on LAPHROAIG
Windowing system distributor 'Microsoft Corp.', version 6.1.7601
Configured using:
 'configure --without-dbus --without-compress-install CFLAGS=-static'

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND NOTIFY ACL GNUTLS LIBXML2 ZLIB
TOOLKIT_SCROLL_BARS

Important settings:
  value of $LANG: DEU
  locale-coding-system: cp1252

Major mode: Debugger

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
  transient-mark-mode: t

Recent messages:
Loading time...done
Entering debugger...
--8<---------------cut here---------------end--------------->8---



Regards,
Achim.
-- 
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+

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