asn1/a_int.c: clean up asn1_get_int64.

Trouble was that integer negation wasn't producing *formally* correct
result in platform-neutral sense. Formally correct thing to do is
-(int64_t)u, but this triggers undefined behaviour for one value that
would still be representable in ASN.1. The trigger was masked with
(int64_t)(0-u), but this is formally inappropriate for values other
than the problematic one. [Also reorder branches to favour most-likely
paths and harmonize asn1_string_set_int64 with asn1_get_int64].]

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3231)