null pointer dereference detection in DMD

[Richard (Rikki) Andrew Cattermole]

For MSVC:

flags: ``/analyze``

```
<source>(5) : warning C6011: Dereferencing NULL pointer 'ptr'. : Lines: 4, 5
ASM generation compiler returned: 0
example.c
<source>(5) : warning C6011: Dereferencing NULL pointer 'ptr'. : Lines: 4, 5
Execution build compiler returned: 0
```