Ensure dhparams can handle X9.42 params in DER

dhparams correctly handles X9.42 params in PEM format. However it failed
to correctly processes them when reading/writing DER format.

Fixes #3102

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3111)
(cherry picked from commit 18d20b5e)