scripts: Don't trigger option hints for short options.

Previously, 'guix install foo -r bar' would crash with a backtrace
because NAME would be #\r (a character instead of a string).

* guix/scripts.scm (parse-command-line)[parse-options-from]: Call
'option-hint' only when NAME is a string.

guix/scripts.scm

 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2014, 2015, 2017, 2018, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015, 2017, 2018, 2019, 2020, 2021, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2014 Deck Pickard <deck.r.pickard@gmail.com>
 ;;; Copyright © 2015, 2016 Alex Kost <alezost@gmail.com>
 ;;; Copyright © 2020 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
@@ -157,7 +157,8 @@ (define (parse-options-from args seeds)
     ;; Actual parsing takes place here.
     (apply args-fold* args options
            (lambda (opt name arg . rest)
-             (let ((hint (option-hint name options)))
+             (let ((hint (and (string? name)      ;not a short option
+                              (option-hint name options))))
                (report-error (G_ "~A: unrecognized option~%") name)
                (when hint
                  (display-hint