g10: Improve handling of no corresponding public key.
* g10/getkey.c (get_seckey): Return G10ERR_NO_PUBKEY when it's not exact match.
In the situation of corrupted .gnupg/ where only private subkey is
available but no corresponding public key of the subkey, the code
returned public primary key which caused mysterious error (for a
user). This fix detects an error earlier.