nat: Allocate the endpoint status dynamically

Allocate the status for an endpoint dynamically. We will support
BSCs with different amount of multiplexes and need to have this
flexibility in the future. Add the proper null checks to the
current users of this code.
diff --git a/openbsc/src/nat/bsc_nat_utils.c b/openbsc/src/nat/bsc_nat_utils.c
index 7b75fa5..0470387 100644
--- a/openbsc/src/nat/bsc_nat_utils.c
+++ b/openbsc/src/nat/bsc_nat_utils.c
@@ -126,6 +126,7 @@
 	conf->token = talloc_strdup(conf, token);
 	conf->nr = nat->num_bsc;
 	conf->nat = nat;
+	conf->number_multiplexes = 1;
 
 	INIT_LLIST_HEAD(&conf->lac_list);