Fix d2i_PrivateKey_ex() to work as documented

Fix d2i_PrivateKey_ex() to work as documented

d2i_PrivateKey(), and thereby d2i_PrivateKey_ex(), is documented to
return keys of the type given as first argument |type|, unconditionally.
Most specifically, the manual says this:

> An error occurs if the decoded key does not match type.

However, when faced of a PKCS#8 wrapped key, |type| was ignored, which
may lead to unexpected results.

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/11787)