Fix an OOB read access in _ksba_dn_to_str.
* src/dn.c (append_utf8_value): Use a straightforward check to fix an off-by-one.
The old fix for the problem from April 2015 had an off-by-one in the
bad encoding handing.
- Fixes-commit: 243d12fdec66a4360fbb3e307a046b39b5b4ffc3
- GnuPG-bug-id: T2344
- Reported-by: Pascal Cuoq
- Signed-off-by: Werner Koch <wk@gnupg.org>