Add missing error handling in h2_rx_data

A failing write on a different stream will set h2->error, which would
cause us to panic here on the following AZ(h2->mailcall).

Fixes: #3040

bin/varnishd/http2/cache_http2_proto.c

@@ -750,6 +750,8 @@ h2_rx_data(struct worker *wrk, struct h2_sess *h2, struct h2_req *r2)
 	Lck_Lock(&h2->sess->mtx);
 	while (h2->mailcall != NULL && h2->error == 0 && r2->error == 0)
 		AZ(Lck_CondWait(h2->cond, &h2->sess->mtx, 0));
+	if (h2->error || r2->error)
+		return (h2->error ? h2->error : r2->error);
 	AZ(h2->mailcall);
 	h2->mailcall = r2;
 	h2->req0->r_window -= h2->rxf_len;