anv: Avoid null ptr dereference

Avoids the sanitizer error:

```
../src/intel/vulkan/anv_instance.c:266:37: runtime error: member access within null pointer of type 'struct anv_instance'
```

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32098>

src/intel/vulkan/anv_instance.c

@@ -263,7 +263,7 @@ PFN_vkVoidFunction anv_GetInstanceProcAddr(
     const char*                                 pName)
 {
    ANV_FROM_HANDLE(anv_instance, instance, _instance);
-   return vk_instance_get_proc_addr(&instance->vk,
+   return vk_instance_get_proc_addr(instance ? &instance->vk : NULL,
                                     &anv_instance_entrypoints,
                                     pName);
 }