Don't panic on worker pool failuers during H2:OU handshake

Fixes	#2937

bin/varnishd/http2/cache_http2_session.c

@@ -303,7 +303,7 @@ h2_ou_session(struct worker *wrk, struct h2_sess *h2,
 		r2->scheduled = 0;
 		h2_del_req(wrk, r2);
 		VSLb(h2->vsl, SLT_Debug, "H2: No Worker-threads");
-		return (h2_ou_rel(wrk, req));
+		return (0);
 	}
 	return (1);
 }

bin/varnishtest/tests/r02937.vtc

+varnishtest "#2937: Panic on OU pool failure"
+
+server s1 {
+	rxreq
+	txresp
+} -start
+
+varnish v1 -vcl+backend {} -start
+
+varnish v1 -cliok "param.set feature +http2"
+varnish v1 -cliok "debug.reqpool.fail F"
+
+client c1 {
+	send "GET / HTTP/1.1\r\n"
+	send "Host: foo\r\n"
+	send "Upgrade: h2c\r\n"
+	send "HTTP2-Settings: AAMAAABkAAQAAP__\r\n"
+	send "\r\n"
+	rxresp
+	expect resp.status == 101
+	expect resp.http.upgrade == h2c
+	expect resp.http.connection == Upgrade
+	txpri
+	expect_close
+} -run