nat: Do not use 0/0 for mux/timeslot by default

0 is a valid timeslot and we should not use it... use
a negative value to be save.
diff --git a/openbsc/src/nat/bsc_sccp.c b/openbsc/src/nat/bsc_sccp.c
index 3906bc9..33142a9 100644
--- a/openbsc/src/nat/bsc_sccp.c
+++ b/openbsc/src/nat/bsc_sccp.c
@@ -99,6 +99,7 @@
 		return -1;
 	}
 
+	bsc_mgcp_clear(conn);
 	llist_add_tail(&conn->list_entry, &bsc->nat->sccp_connections);
 	counter_inc(bsc->cfg->stats.sccp.conn);
 	counter_inc(bsc->cfg->nat->stats.sccp.conn);