Avoid subtracting form .length

[Dom DiSc]

I _very_ often use this pattern:

```
fun(ref int[] a)
{
    assert(a.length && a.length<=100);
    int[100] b;
    b[0 .. a.length-1] = a[];
    b[a.length .. 100] = 5;
}
```

I consider this perfectly safe, but DScanner gives warnings for 
this, no matter if a check is present or not.
What's the recommended fix for this? I have no idea what else to 
use.