bug#68593: 29.1; comment-dwim: Args out of range: "", 0, 1

[Frank Ruben]

In GNU Emacs 29.1 (build 2, x86_64-w64-mingw32) of 2023-08-02 built on AVALON
Windowing system distributor 'Microsoft Corp.', version 10.0.19045
System Description: Microsoft Windows 10 Pro (v10.0.2009.19045.3930)

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 'configure --with-modules --without-dbus --with-native-compilation=aot
 --without-compress-install --with-tree-sitter CFLAGS=-O2'

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(NATIVE_COMP present but libgccjit not available)

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Memory information:
((conses 16 1328082 122098)
 (symbols 48 35374 0)
 (strings 32 187080 7259)
 (string-bytes 1 5454999)
 (vectors 16 81243)
 (vector-slots 8 1843247 151854)
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 (intervals 56 162466 825)
 (buffers 984 88))

When in c-ts-mode.el and when having '(setq comment-end " ")' and when I then
then run e.g. 'comment-dwim' or simpler '(comment-normalize-vars)', I'm getting
'comment-dwim: Args out of range: "", 0, 1'.

The error comes from the two 'substring' calls in this s-sexp - where the code
already contains my "fix" - both calls to 'substring' have been guarded by
'(if (string-equal ce "") "" ...)'.

There are surely smarter ways to fix that - if one knows what is supposed to
happen here :). But maybe that fix gives you a hint to the appropriate
solution.

(let ((ce (if (string= "" comment-end) "\n"
 (comment-string-strip comment-end t t))))
        (setq-local comment-end-skip
                    ;; We use [ \t] rather than \s- because we don't want to
                    ;; remove ^L in C mode when uncommenting.
                    (concat "[ \t]*\\(\\s>" (if comment-quote-nested "" "+")
                            "\\|" (regexp-quote (if (string-equal ce "") "" (substring ce 0 1)))
                            (if (and comment-quote-nested (<= (length ce) 1)) "" "+")
                            (regexp-quote (if (string-equal ce "") "" (substring ce 1)))
                            "\\)")))