Fix int64 test of t_4bytes_4_neg

{ 0x80, 0x00, 0x00, 0x00 } decoded isn't (positive) 0x80000000,
it's (negative) INT32_MIN.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3160)