bug#31557: 27.0.50; SES hangs on save when 'delete-trailing-whitespace' is in 'before-save-hook'

[Filipp Gunbin]

emacs -Q
M-: (add-hook 'before-save-hook #'delete-trailing-whitespace)
C-x C-f ~/tmp/1.ses (file does not exist)
C-x C-s

At this point emacs hangs, echo area shows "Saving file .../1.ses".  

With `toggle-error-on-quit', I see the following backtrace:

  re-search-forward("\\s-$" nil t)
  delete-trailing-whitespace()
  run-hooks(before-save-hook)
  basic-save-buffer(t)
  save-buffer(1)
  funcall-interactively(save-buffer 1)
  call-interactively(save-buffer nil nil)
  command-execute(save-buffer)



In GNU Emacs 27.0.50 (build 10, x86_64-apple-darwin17.5.0)
 of 2018-05-14 built on fgunbin.playteam.ru
Repository revision: 15fa8de1ae3228413fde95e583008d9b9f19e7c7
System Description:  Mac OS X 10.13.4

Configured using:
 'configure CC=/usr/bin/gcc --without-all --without-ns --with-dbus
 --with-file-notification=no --with-gnutls --with-modules --with-threads
 --with-xml2 --with-zlib --without-x'

Configured features:
GNUTLS LIBXML2 ZLIB MODULES THREADS