Fix double increment in DN parser while counting hexdigits.

* src/dn.c (parse_rdn): Turn for-loop into a while-loop.

This bug could be used to mount a DoS due to an over-read and thus
potentially reading from non-allocated memory.

(from 2002)

• GnuPG-bug-id: T8104
• Fixes-commit: be159990669b811050344c50c6ac87a8e705fdfb