Preventing nested struct destructor accessing stack frame

[Nick Treleaven via Digitalmars-d-learn]

This code segfaults when the GC calls the dtor after the unittest 
succeeds:

```d
unittest
{
    int i;
    struct S
    {
        ~this() { i++; }
    }
    (*new S).destroy;
}
```
It seems destroy clears the context pointer. Is there a way to 
test if the context pointer is null in the dtor, to prevent the 
increment?