BN_set_negative
void BN_set_negative(BIGNUM */*a*/, int /*n*/);
BN_set_negative(nbytes, 0);
BN_set_negative(t1, 1);
BN_set_negative(nbytes, 1);
BN_set_negative(n->number, !BN_is_negative(n->number));
BN_set_negative(v, 0);