Make EVP_PKEY_asn1_add0() stricter about its input

It turns out that the strictness that was implemented in
EVP_PKEY_asn1_new() (see Github openssl/openssl#6880) was badly placed
for some usages, and that it's better to do this check only when the
method is getting registered.

Fixes #7758

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7847)