[IPV6]: Make sure fib6_rule_lookup doesn't return NULL

The callers of fib6_rule_lookup don't expect it to return NULL,
therefore it must return ip6_null_entry whenever fib_rule_lookup fails.
Signed-off-by: Ville Nuorvala <vnuorval@tcs.hut.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/fib6_rules.c

@@ -63,7 +63,11 @@ struct dst_entry *fib6_rule_lookup(struct flowi *fl, int flags,
 	if (arg.rule)
 		fib_rule_put(arg.rule);
 
+	if (arg.result)
 		return (struct dst_entry *) arg.result;
+
+	dst_hold(&ip6_null_entry.u.dst);
+	return &ip6_null_entry.u.dst;
 }
 
 static int fib6_rule_action(struct fib_rule *rule, struct flowi *flp,