bug#62290: Error when handling invalid unicode with suspendable ports

Here's a simple reproducer:

  (use-modules (ice-9 binary-ports)
               (ice-9 suspendable-ports)
               (rnrs bytevectors))

  (define (test)
    (let* ((sequence
             '(#xf4 #xa4 #xbd #xa4))
           (p (open-bytevector-input-port
               (u8-list->bytevector sequence))))
      (set-port-encoding! p "UTF-8")
      (set-port-conversion-strategy! p 'substitute)
      (peek (read-char p))))

  (test)

  (install-suspendable-ports!)

  (test)


If you run it, it outputs #\� as expected the first time, but then using
suspendable ports, it raises an exception. The behaviour should be the
same.


;;; (#\�)
Backtrace:
In ice-9/boot-9.scm:
  1752:10  8 (with-exception-handler _ _ #:unwind? _ # _)
In unknown file:
           7 (apply-smob/0 #<thunk 7f3f09cbe300>)
In ice-9/boot-9.scm:
    724:2  6 (call-with-prompt ("prompt") #<procedure 7f3f09ccb320 …> …)
In ice-9/eval.scm:
    619:8  5 (_ #(#(#<directory (guile-user) 7f3f09cc1c80>)))
In ice-9/boot-9.scm:
   2836:4  4 (save-module-excursion #<procedure 7f3f09cb2300 at ice-…>)
  4388:12  3 (_)
In /home/chris/Projects/Guile/guile/bad-unicode.scm:
    12:10  2 (test)
In ice-9/suspendable-ports.scm:
   591:33  1 (read-char _)
   499:12  0 (peek-char-and-next-cur/utf8 _ _ _ _)

ice-9/suspendable-ports.scm:499:12: In procedure peek-char-and-next-cur/utf8:
In procedure integer->char: Argument 1 out of range: 1199972