nat: Store the config in the connection instead of the lac

This allows that we can print the Nr. next to the lac
and it allows us to change the lac at runtime without
reconnecting the BSC.
diff --git a/openbsc/src/nat/bsc_nat_utils.c b/openbsc/src/nat/bsc_nat_utils.c
index 81d4de9..eddcaff 100644
--- a/openbsc/src/nat/bsc_nat_utils.c
+++ b/openbsc/src/nat/bsc_nat_utils.c
@@ -115,7 +115,9 @@
 	for (i = 1; i < data_length - 1; i += 2) {
 		unsigned int _lac = ntohs(*(unsigned int *) &data[i]);
 		llist_for_each_entry(bsc, &nat->bsc_connections, list_entry) {
-			if (!bsc->authenticated || _lac != bsc->lac)
+			if (!bsc->cfg)
+				continue;
+			if (!bsc->authenticated || _lac != bsc->cfg->lac)
 				continue;
 
 			return bsc;