bug#32319: 27.0.50; flyspell-mode error on start

[Eric Abrahamsen <eric@ericabrahamsen.net>]

Turning on flyspell-mode started giving me an error a few days ago, on
two different machines I use. Both are Arch linux, both running Emacs
master.

My only ispell customizations:

(setq ispell-program-name (executable-find "hunspell"))
(setq ispell-personal-dictionary "~/.aspell.en_US.pws")

Starting flyspell-mode gives me:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  string-match("," nil 0)
  split-string(nil "," t)
  ispell-parse-hunspell-affix-file(nil)
  ispell-hunspell-fill-dictionary-entry(nil)
  ispell-start-process()
  ispell-init-process()
  ispell-buffer-local-words()
  ispell-accept-buffer-local-defs()
  flyspell-accept-buffer-local-defs(force)
  flyspell-mode-on()

From "emacs -Q", setting `ispell-program-name' to (executable-find
"hunspell") (the first line of my customizations above) is sufficient to
trigger the error.

`ispell-current-dictionary' seems to be nil during the whole process,
which causes the eventual error.

The Hunspell version on Arch is "1.6.2", which is the value of
`ispell-really-hunspell'.

Hope that's enough information!

In GNU Emacs 27.0.50 (build 37, x86_64-pc-linux-gnu, GTK+ Version 3.22.30)
 of 2018-07-30 built on clem
Repository revision: 63ef79329935b790b9c8107125bce66e1f272c2e
Windowing system distributor 'The X.Org Foundation', version 11.0.12000000
System Description: Arch Linux

Recent messages:
Entering debugger...
Debug on Error disabled globally
Mark set
Mark activated
Back to top level
Mark set
Quit [2 times]
Mark saved where search started
"/usr/bin/hunspell"
"1.6.2"