Avoid a NULL ptr deref if group is not set

We should only copy parameters and keys if the group is set. Otherwise
they don't really make any sense. Previously we copied the private key
regardless of whether the group was set...but if it wasn't a NULL ptr
deref could occur. It's unclear whether we could ever get into that
situation, but since we were already checking it for the public key we
should be consistent.

Reviewed-by: Rich Salz <rsalz@openssl.org>