dirmngr: Fix error return for ADNS.
* dirmngr/dns-stuff.c (resolve_name_adns): Use RET for return value.
There are cases where libadns returns an error without setting the
variable ERRNO.
- GnuPG-bug-id: T2745
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>