Q: Given the following code slice:
```
1 static int __fib6_rule_action(struct fib_rule *rule, struct flowi *flp,
2 			      int flags, struct fib_lookup_arg *arg)
5 	struct flowi6 *flp6 = &flp->u.ip6;
7 	struct fib6_table *table;
8 	struct net *net = rule->fr_net;
9 	pol_lookup_t lookup = arg->lookup_ptr;
11 	u32 tb_id;
13 	switch (rule->action) {
18 		rt = net->ipv6.ip6_null_entry;
23 		rt = net->ipv6.ip6_blk_hole_entry;
27 		rt = net->ipv6.ip6_prohibit_entry;
31 	tb_id = fib_rule_get_table(rule, arg);
32 	table = fib6_get_table(net, tb_id);
33 	if (!table) {
38 	rt = pol_lookup_func(lookup,
40 	if (rt != net->ipv6.ip6_null_entry) {```
which has a CWE-476 vulnerability at line:
```
40 	if (rt != net->ipv6.ip6_null_entry) {
```
Please analyze the root cause of the vulnerability.