bug#20448: 25.0.50; 25.0.50; `ispell-async-processp' set to nil causes an error from (ispell-start-process)

[Michael Pontus <m.pontus@gmail.com>]

Steps to reproduce:

1. Start emacs -Q
2. M-: (setq ispell-async-processp t)
3. M-: (ispell-start-process)

Backtrace:

    Debugger entered--Lisp error: (wrong-type-argument stringp nil)
      file-accessible-directory-p(nil)
      ispell-start-process()
      eval((ispell-start-process) nil)
      eval-expression((ispell-start-process) 1)
      funcall-interactively(eval-expression (ispell-start-process) 1)
      call-interactively(eval-expression nil nil)
      command-execute(eval-expression)

Following excerpt from ispell.el indicates that function
`ispell-start-process' may return one of two types of values: process
object or t, depending on the value of user-option in question.

        (if ispell-async-processp
            (let ((process-connection-type ispell-use-ptys-p))
              (apply 'start-process
                     "ispell" nil ispell-program-name
                     "-a"                   ; Accept single input lines.
                     ;; Make root/affix combos not in dict.
                     ;; hunspell -m option means different.
                     (if ispell-really-hunspell "" "-m")
                     args))
          (setq ispell-cmd-args args
                ispell-output-buffer (generate-new-buffer " *ispell-output*")
                ispell-session-buffer (generate-new-buffer " *ispell-session*"))
          (ispell-send-string "\032\n")        ; so Ispell prints version and exits
          t)

In GNU Emacs 25.0.50.2 (x86_64-unknown-linux-gnu, GTK+ Version 3.14.9)
 of 2015-04-23 on gentoo
Windowing system distributor `The X.Org Foundation', version 11.0.11604000
Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
ACL GNUTLS LIBXML2 FREETYPE XFT ZLIB

Important settings:
  value of $LANG: en_US.utf8
  locale-coding-system: utf-8-unix

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

Recent messages:
read--expression: Command attempted to use minibuffer while in minibuffer
nil
Entering debugger...
Mark set [2 times]
Quit
Making completion list...
Quit
You can run the command `info-emacs-bug' with M-x inf-b RET
Quit
mouse-2, RET: find function's definition

Load-path shadows:
None found.

Features:
(shadow sort gnus-util mail-extr jka-compr info 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 help-mode easymenu
debug ispell mule-util 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 cl-preloaded 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
dbusbind gfilenotify dynamic-setting system-font-setting
font-render-setting move-toolbar gtk x-toolkit x multi-tty
make-network-process emacs)

Memory information:
((conses 16 88756 8278)
 (symbols 48 19099 0)
 (miscs 40 83 232)
 (strings 32 16546 5309)
 (string-bytes 1 419420)
 (vectors 16 11192)
 (vector-slots 8 404782 4985)
 (floats 8 95 271)
 (intervals 56 276 0)
 (buffers 976 16)
 (heap 1024 22138 747))