g10: Fix checking key for signature validation.
* g10/sig-check.c (signature_check2): Not only subkey, but also primary key should have flags.valid=1.
(backport of master
commit 6f284e6ed63f514b15fe610f490ffcefc87a2164)
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>