Fix encoding of invalid utf-8 strings in dn.c

* src/dn.c (append_quoted, append_atv): Use snprintf.
(append_utf8_value): Fix invalid encoding handling.

An invalid utf-8 encoding will make the loop in append_utf8_value run
once more with N > length which is not found by the termination
condition and only the former assert terminates the process if the byte
following the bad encoding has the high bit cleared. This will lead
to a read access out of bounds.

The patch removes the assert and fixes the handling of bad encoding.
Due to the new quoting the output of a badly encoded utf-8 string will
be different than in previous versions.

Replacing sprintf is only for cosmetic reasons.

Use "gpgsm --verify FILE" to exhibit the problem. FILE is
-----BEGIN PGP ARMORED FILE-----

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MDAAMDEwAgEwMDAwMDEwMDAGA1UEAwwB/4AwMDAwMDAwMDAwMDAwMDAwMDAwMDAw
MDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAw

NJTr

-----END PGP ARMORED FILE-----

• Reported-by: Hanno Böck
• Signed-off-by: Werner Koch <wk@gnupg.org>