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

The function `of_find_property` does not check if the input parameter `np` (device_node pointer) is NULL before passing it to `__of_find_property` on line 9. If `np` is NULL, it could lead to a NULL pointer dereference when `__of_find_property` attempts to access the members or data of the device_node structure.

To prevent this vulnerability, the function should include a NULL check for the `np` parameter at the beginning of the function, before any operations are performed using it. For example:

```c
if (np == NULL) {
    return NULL; // or handle the error appropriately
}
```

This check would ensure that the function safely handles cases where a NULL pointer is passed as the device_node, preventing potential crashes or undefined behavior due to NULL pointer dereference.