network: add L_WARN for known network lookup failure

When updating the network ranking there was a potential out of bounds
array access. The condition was if known_network_offset returned a
negative value, indicating the known network was not found. Since
network->info is only set for known networks this should not ever
happen as network->info is checked prior.

Though this is likely impossible, knownnetworks is complex enough that
its better to just be paranoid and put an L_WARN_ON to check the
return.

src/network.c

@@ -1376,6 +1376,8 @@ void network_rank_update(struct network *network, bool connected)
 	if (network->info->connected_time != 0) {
 		int n = known_network_offset(network->info);
 
+		L_WARN_ON(n < 0);
+
 		if (n >= (int) L_ARRAY_SIZE(rankmod_table))
 			n = L_ARRAY_SIZE(rankmod_table) - 1;