g10: Fix checking key for signature validation.
* g10/sig-check.c (check_signature2): Not only subkey, but also primary key should have flags.valid=1.
The tweak of gpgv in e32c575e0f3704e7563048eea6d26844bdfc494b only makes
sense with this change.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>