asn1_item_embed_new(): don't free an embedded item

An embedded item wasn't allocated separately on the heap, so don't
free it as if it was.

Issue discovered by Pavel Kopyl

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4572)

(cherry picked from commit 590bbdfd)