Handle pointers to union

Unions are declared as normal global variables initialized to null values. The test cases like the below are not modeled,

```c
union U {
  int *f;
}u, *v
int main(){
  int a;
  u.f = &a // this works as it uses a GEP
  v -> f = &a // this does not work; fix this
  return 0;
}
```
