negating without checking if zero

For example:

```c++
alt_bn128_G1 alt_bn128_G1::operator-() const
{
    return alt_bn128_G1(this->X, -(this->Y), this->Z);
}
```

`-G1::zero()` thus has an invalid jacobian representation. I doubt any code outside of this will be reading Y without running `to_affine_coordinates` first (which overwrites the invalid Y coordinate) so this *probably* won't manifest as a bug elsewhere.