Free up passed ASN.1 structure if reused.

Change the "reuse" behaviour in ASN1_item_d2i: if successful the old
structure is freed and a pointer to the new one used. If it is not
successful then the passed structure is untouched.

Exception made for primitive types so ssl_asn1.c still works.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Emilia Käsper <emilia@openssl.org>

Conflicts:
	doc/crypto/d2i_X509.pod