http2: clear an in-progress request when deleting it

otherwise, for the modified vtc, a continuation is still expected when another
stream is opened:

1031 SessError       c H2: expected continuation but  received HEADERS on stream 3
1031 Debug           c H2 CLEANUP H2CE_PROTOCOL_ERROR

Partly addresses #4283

Author: nigoroll

bin/varnishd/http2/cache_http2_proto.c

@@ -192,6 +192,8 @@ h2_del_req(struct worker *wrk, struct h2_req *r2)
 	--h2->refcnt;
 	/* XXX: PRIORITY reshuffle */
 	VTAILQ_REMOVE(&h2->streams, r2, list);
+	if (r2->req == h2->new_req)
+		h2->new_req = NULL;
 	Lck_Unlock(&sp->mtx);
 
 	assert(!WS_IsReserved(r2->req->ws));

bin/varnishtest/tests/t02023.vtc

@@ -130,4 +130,8 @@ client c1 {
 		rxrst
 		expect rst.err == PROTOCOL_ERROR
 	} -run
+	stream 3 {
+		txreq
+		rxresp
+	} -run
 } -run