nat: In the case of losing the MSC, reset all endpoints

When losing the SCCP connection make sure that we free all
endpoints. The disconnection of the BSC should already make
sure they are closed but this makes sure everything is
properly reset.
diff --git a/openbsc/src/nat/bsc_nat.c b/openbsc/src/nat/bsc_nat.c
index a868957..f69a994 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -304,6 +304,7 @@
 	llist_for_each_entry_safe(bsc, tmp, &nat->bsc_connections, list_entry)
 		remove_bsc_connection(bsc);
 
+	bsc_mgcp_free_endpoints(nat);
 	bsc_msc_schedule_connect(con);
 }