Questions about some clear functions usage #659

I saw a lot of clear functions used to clear local scalar, ge or gej variable in the last step of function, does this necessary? I thought the local variables should be no longer exists(or used) after function quit. Is this a manual destruct for local variable? Because it takes longer time(maybe just a small longer time) than just leave them alone.

Here are example codes.

static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context *ctx, secp256k1_gej *r, const secp256k1_scalar *gn) {
    secp256k1_ge add;
    secp256k1_scalar gnb;
    int bits;
    // ........
    secp256k1_ge_clear(&add);
    secp256k1_scalar_clear(&gnb);
}

They (optimistically) try to wipe potentially secret data.

See #636

Thanks for reply. Solve the questions lasted for days.