Ensure that only the rxthread gets to use h2->cond in h2_send_get

Future-proofing to avoid mistakenly introducing another race down the
line.

bin/varnishd/http2/cache_http2_send.c

@@ -45,6 +45,8 @@ h2_send_get(struct worker *wrk, struct h2_sess *h2, struct h2_req *r2)
 	CHECK_OBJ_NOTNULL(wrk, WORKER_MAGIC);
 
 	Lck_AssertHeld(&h2->sess->mtx);
+	if (&wrk->cond == h2->cond)
+		ASSERT_RXTHR(h2);
 	r2->wrk = wrk;
 	VTAILQ_INSERT_TAIL(&h2->txqueue, r2, tx_list);
 	while (VTAILQ_FIRST(&h2->txqueue) != r2)