Fix NULL pointer dereference in `asn1_ex_i2c()`, crypto/asn1/tasn_enc.c

Fix NULL pointer dereference in `asn1_ex_i2c()`, crypto/asn1/tasn_enc.c

Adds handling of V_ASN1_UNDEF to avoid NULL dereference
in case ASN1 structure contains an element of type ASN1_TYPE
without initializing its value (i.e. default constructed)

CLA: trivial

Signed-off-by: Andrey Tsygunka <aitsygunka@yandex.ru>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27100)