Fix msan bug

test/core/util/passthru_endpoint.c

@@ -149,6 +149,7 @@ void grpc_passthru_endpoint_create(grpc_endpoint **client,
                                    grpc_endpoint **server) {
   passthru_endpoint *m = gpr_malloc(sizeof(*m));
   m->halves = 2;
+  m->shutdown = 0;
   half_init(&m->client, m);
   half_init(&m->server, m);
   gpr_mu_init(&m->mu);