To address the CWE-476 (NULL Pointer Dereference) vulnerability in the given code slice, specifically on line 20, here are five possible patches:

Patch 1: Add a NULL check before dereferencing
```c
19 	if (neigh) {
20 		if (neigh->ha) {
21 			memcpy(nexthop_mac, neigh->ha, ETH_ALEN);
22 			*uses_gateway = rt->rt_uses_gateway;
23 			return 0;
24 		}
25 	}
```

Patch 2: Use a safe memory copy function
```c
19 	if (neigh) {
20 		if (neigh->ha) {
21 			memcpy_safe(nexthop_mac, neigh->ha, ETH_ALEN);
22 			*uses_gateway = rt->rt_uses_gateway;
23 			return 0;
24 		}
25 	}
```

Patch 3: Use a ternary operator to handle NULL case
```c
19 	if (neigh) {
20 		memcpy(nexthop_mac, neigh->ha ? neigh->ha : (unsigned char[]){0}, ETH_ALEN);
21 		*uses_gateway = rt->rt_uses_gateway;
22 		return 0;
23 	}
```

Patch 4: Use a macro for safe dereferencing
```c
#define SAFE_DEREF(ptr, default) ((ptr) ? (ptr) : (default))

19 	if (neigh) {
20 		memcpy(nexthop_mac, SAFE_DEREF(neigh->ha, (unsigned char[]){0}), ETH_ALEN);
21 		*uses_gateway = rt->rt_uses_gateway;
22 		return 0;
23 	}
```

Patch 5: Use a helper function for safe memory copy
```c
static inline void safe_memcpy(void *dest, const void *src, size_t n) {
    if (src) {
        memcpy(dest, src, n);
    } else {
        memset(dest, 0, n);
    }
}

19 	if (neigh) {
20 		safe_memcpy(nexthop_mac, neigh->ha, ETH_ALEN);
21 		*uses_gateway = rt->rt_uses_gateway;
22 		return 0;
23 	}
```

Each of these patches aims to prevent a NULL pointer dereference by either checking for NULL before accessing `neigh->ha` or providing a safe alternative when `neigh->ha` is NULL.