[cpp] type 'null' does not have an overloaded member 'operator ->'

``` haxe
class Main {
public static function main() {}

var def(default, set):Def;

function destroy() {
def = null;
}

inline function set_def(newDef:Def):Def {
newDef.apply();
return newDef;
}
}

class Def {
public function apply() null;
}
```

```
./src/Main.cpp(32): error C2819: type 'null' does not have an overloaded member 'operator ->'
c:\haxetoolkit\haxe\lib\hxcpp\git\include\null.h(66): note: see declaration of 'null'
./src/Main.cpp(32): note: did you intend to use '.' instead?
./src/Main.cpp(32): error C2039: 'apply': is not a member of 'null'
c:\haxetoolkit\haxe\lib\hxcpp\git\include\null.h(66): note: see declaration of 'null'
```

Interesting dump:

``` haxe
destroy(method) : Void -> Void

= function() = {
null.apply();
}
```

Regression.