asn1_item_embed_new(): don't free an embedded item

The previous change with this intention didn't quite do it.  An
embedded item must not be freed itself, but might potentially contain
non-embedded elements, which must be freed.

So instead of calling ASN1_item_ex_free(), where we can't pass the
embed flag, we call asn1_item_embed_free() directly.

This changes asn1_item_embed_free() from being a static function to
being a private non-static function.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/4579)