sm: Fix a bug in the rfc2253 parser

* sm/certdump.c (parse_dn_part): Fix parser flaw.

This could in theory result in reading bytes after a after Nul in a
string and thus possible segv on unallocated memory or reading other
parts of the memory. However, it is harmless because the rfc2253
strings have been constructed by libksba.

• GnuPG-bug-id: T5037
• Signed-off-by: Werner Koch <wk@gnupg.org>