Fix PEM_write_bio_PrivateKey_traditional() to not output PKCS#8

Fix PEM_write_bio_PrivateKey_traditional() to not output PKCS#8

PEM_write_bio_PrivateKey_traditional() uses i2d_PrivateKey() to do the
actual encoding to DER.  However, i2d_PrivateKey() is a generic
function that will do what it can to produce output according to what
the associated EVP_PKEY_ASN1_METHOD offers.  If that method offers a
function 'old_priv_encode', which is expected to produce the
"traditional" encoded form, then i2d_PrivateKey() uses that.  If not,
i2d_PrivateKey() will go on and used more modern methods, which are
all expected to produce PKCS#8.

To ensure that PEM_write_bio_PrivateKey_traditional() never produces
more modern encoded forms, an extra check that 'old_priv_encode' is
non-NULL is added.  If it is NULL, an error is returned.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/12728)