ge25519_unpackneg_vartime
int ge25519_unpackneg_vartime(ge25519 *r, const unsigned char p[32]);
if (ge25519_unpackneg_vartime(&get1, pk)) return -1;
if (ge25519_unpackneg_vartime(&get1,pk)) goto badsig;
int ge25519_unpackneg_vartime(ge25519 *r, const unsigned char p[32]);