(CVE-2020-13645) GTlsClientConnection silently ignores unset server identity

When the server-identity property of GTlsClientConnection is unset, the documentation says we need to fail the certificate verification with G_TLS_CERTIFICATE_BAD_IDENTITY:

If the G_TLS_CERTIFICATE_BAD_IDENTITY flag is set in “validation-flags”, this object will be used to determine the expected identify of the remote end of the connection; if “server-identity” is not set, or does not match the identity presented by the server, then the G_TLS_CERTIFICATE_BAD_IDENTITY validation will fail.

This is important because otherwise, it's easy for applications to fail to specify server identity. When server identity is missing, we check the validity of the TLS certificate but do not check if it corresponds to the expected server. That is, evil.com can present a valid certificate issued to evil.com, and we will happily accept it for paypal.com.

This was discovered in balsa#34 (closed).