eckey_priv_encode(): Call ASN1_STRING_free() only on an ASN1_STRING

Also ASN1_OBJECT_free() never needs to be called on objects returned
from OBJ_nid2obj().

Fixes #19138

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19367)