The root cause of the CWE-476 (NULL Pointer Dereference) vulnerability in the given code slice is:

The function PCI_DN() does not check if the input parameter 'dn' is NULL before dereferencing it to access the 'data' member. If 'dn' is NULL, attempting to access 'dn->data' will result in a NULL pointer dereference, which can lead to a program crash or undefined behavior.

Specifically:
1. The function assumes that 'dn' is a valid, non-NULL pointer.
2. It directly accesses 'dn->data' without any prior NULL check.
3. If 'dn' is NULL, the attempt to access 'data' will cause a NULL pointer dereference.

To fix this vulnerability, a NULL check should be added before accessing 'dn->data'. For example:

```c
struct pci_dn *PCI_DN(dn) {
    if (dn == NULL) {
        return NULL;
    }
    return ((struct pci_dn *) (dn)->data);
}
```

This check ensures that the function handles NULL input safely, preventing the potential NULL pointer dereference.