Fix test/asn1_encode_test.c, ASN1_LONG_DATA used inappropriately

It's sheer luck that this was used for the first field only which also
has the same type in all data structures, so the offsets were never wrong

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3127)