It is possible to implement an rdp client that accepts certificates by
fingerprint by using VerifyCertificateEx. In case the server uses a
certificate without subject (which, apparently, is not mandated by X509)
freerdp_certificate_data_load_cache fails and the certificate is refused
even before calling VerifyCertificateEx. This commit changes
freerdp_certificate_data_load_cache to consider that missing subject is
the same as an empty string.
Also downgrade the log message complaining about missing subject and
issuer to a warning.
68b1614b66