bug#43326: [3.0.x] Regression: Compilation error involving strings

[Erik Dominikus <erik.dominikus71@gmail.com>]

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Dear author(s) of the Guile 3.0.x compiler,

Background: I would like to report a problem that I found while trying
to write a simple web server with get-string-n, sxml simple, and web
server.

I have traced and reduced the problem into the minimal example below.

Some tested versions:
- affected: 3.0.2 and 3.0.4
- not affected: 2.0.13

How to reproduce:
- Download the attached compile-error.scm into your working directory.
- Run in Bash: GUILE_AUTO_COMPILE=0 guile -s compile-error.scm

Expected result: #t

Actual result: #f

Thank you.

Best regards,

Erik

[-- Attachment #2: compile-error.scm --]
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;; GUILE_AUTO_COMPILE=0 guile -s compile-error.scm
(define code '(lambda ()
                (define a (string #\x #\y))
                (define b (substring/shared a 0 1))
                (string-ref b 0)))
(define compiled (compile code))
(define interped (eval code (interaction-environment)))
(write (equal? (compiled) (interped))) ;; #\nul vs #\x
(newline)