Fix returning new signatures when there are none.

* src/sign.c (gpgme_op_sign_result): Test that invalid and valid
signatures add up to gpgme_signers_count().

When invalid and valid signatures do not equal gpgme_signers_count() it
means that there was a bad passphrase during signing after the first
signer. This leaves the result.signatures from previous signers intact
which isn't correct since gpg will report:

gpg: number of one-pass packets does not match number of signature
packets
gpg: can't handle this ambiguous signature data

during verify. So when this happens append the valid signatures to the
.invalid_signers list with .reason set to GPG_ERR_GENERAL.