bug#38255: 27.0.50; Indentation error-out in css-mode

[Lars Ingebrigtsen <larsi@gnus.org>]

Open a.css and type:

foo {
}
}

The following backtrace is triggered.

Debugger entered--Lisp error: (wrong-type-argument number-or-marker-p nil)
  smie-next-sexp(#f(compiled-function () #<bytecode 0x155d7de91a0d>) #f(compiled-function (n) #<bytecode 0x155d7f00a465>) #<subr car> #f(compiled-function (x) "Return the car of the cdr of X." #<bytecode 0x1ffc61806017>) "}")
  smie-backward-sexp("}")
  smie-indent-keyword()
  run-hook-with-args-until-success(smie-indent-keyword)
  smie-indent-calculate()
  smie-indent-line()
  indent-according-to-mode()
  electric-indent-post-self-insert-function()
  self-insert-command(1 125)
  funcall-interactively(self-insert-command 1 125)
  call-interactively(self-insert-command nil nil)
  command-execute(self-insert-command)



In GNU Emacs 27.0.50 (build 52, x86_64-pc-linux-gnu, GTK+ Version 3.24.5)
 of 2019-11-17 built on marnie
Repository revision: 3fdc36eecb3ab468eb0a55cc09a176ab503d31c3
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: Debian GNU/Linux 10 (buster)


-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no