[ec_asn1.c] Avoid injecting seed when built-in matches

[ec_asn1.c] Avoid injecting seed when built-in matches

An unintended consequence of https://github.com/openssl/openssl/pull/9808
is that when an explicit parameters curve is matched against one of the
well-known builtin curves we automatically inherit also the associated
seed parameter, even if the input parameters excluded such
parameter.

This later affects the serialization of such parsed keys, causing their
input DER encoding and output DER encoding to differ due to the
additional optional field.

This does not cause problems internally but could affect external
applications, as reported in
https://github.com/openssl/openssl/pull/9811#issuecomment-536153288

This commit fixes the issue by conditionally clearing the seed field if
the original input parameters did not include it.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/10140)