[6.0] 2438f21b4 h2: Avoid sending duplicate rst_stream frames on Pool_Task failure

[Dridi Boukelmoune]

commit 2438f21b4833c69fc35e13d32180a37a6b0fc688
Author: Dag Haavi Finstad <daghf at varnish-software.com>
Date:   Fri Jun 15 16:07:37 2018 +0200

    h2: Avoid sending duplicate rst_stream frames on Pool_Task failure

diff --git a/bin/varnishd/http2/cache_http2_proto.c b/bin/varnishd/http2/cache_http2_proto.c
index 729f0a4ee..7001b60fc 100644
--- a/bin/varnishd/http2/cache_http2_proto.c
+++ b/bin/varnishd/http2/cache_http2_proto.c
@@ -603,6 +603,7 @@ h2_end_headers(struct worker *wrk, struct h2_sess *h2,
 	r2->scheduled = 1;
 	if (Pool_Task(wrk->pool, &req->task, TASK_QUEUE_STR) != 0) {
 		r2->scheduled = 0;
+		r2->state = H2_S_CLOSED;
 		return (H2SE_REFUSED_STREAM); //rfc7540,l,3326,3329
 	}
 	return (0);