nat: Close all endpoints used by a BSC when the BSC is gone

Cleanup all endpoints that belonged to a given BSC. This is
one part of the cleanup, the other is to bring down the SCCP
link properly.
diff --git a/openbsc/src/nat/bsc_nat.c b/openbsc/src/nat/bsc_nat.c
index 9fb4f02..06c130b 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -354,6 +354,9 @@
 		sccp_connection_destroy(sccp_patch);
 	}
 
+	/* close endpoints allocated by this BSC */
+	bsc_mgcp_clear_endpoints_for(connection);
+
 	talloc_free(connection);
 }