Ensure signatures exist when verifying multipart/signed emails. (closes #3881).

Ensure signatures exist when verifying multipart/signed emails. (closes #3881).

TAKAHASHI Tamotsu reported that when gpg2 isn't in PATH, the
gpgme_op_verify() won't return an error, but instead will return a
result with no signatures.

verify_one() was only returning an error if a signature actually
failed, so in this case the function was defaulting to returning
success.

Other callers of gpgme_op_verify() check to make sure the
result->signatures exist before processing signatures.  Add a check
for verify_one() too.