The ESI_VDP would double-close sessions when a flush error happened

before an ESI include at one level, and then again at a higher level.

Fixed by not closing the session in this case, as that will be taken
care of at the end of delivery anyways when V1L_FlushRelease is
called.

Fixes: 		#1737
Submitted by:	Martin

bin/varnishd/cache/cache_esi_deliver.c

@@ -317,7 +317,6 @@ VDP_ESI(struct req *req, enum vdp_action act, void **priv,
 				r = (void*)strchr((const char*)q, '\0');
 				AN(r);
 				if (VDP_bytes(req, VDP_FLUSH, NULL, 0)) {
-					SES_Close(req->sp, SC_REM_CLOSE);
 					ecx->p = ecx->e;
 					break;
 				}

bin/varnishtest/tests/r01737.vtc

+varnishtest "#1737 - ESI sublevel session close"
+
+# Build a esi request tree that fails on flush before include at two different
+# levels. Synchronize a client close after the response headers have been
+# received by the client. This produces write erros for the body parts in all
+# fragments.
+server s1 {
+	rxreq
+	txresp -body {<esi:include src="/bar"/><esi:include src="/bar"/>}
+	sema r1 sync 2
+
+	rxreq
+	delay 1
+	txresp -body {<esi:include src="/baz"/>22<esi:include src="/baz"/>}
+
+	rxreq
+	txresp -body {1}
+
+	rxreq
+	expect req.url == "/check"
+	rxresp
+} -start
+
+varnish v1 -vcl+backend {
+	sub vcl_backend_response {
+		set beresp.do_esi = true;
+	}
+} -start
+
+# Send request, read response headers then close connection
+client c1 {
+	txreq
+	rxresp -no_obj
+	sema r1 sync 2
+} -run
+
+delay 3
+
+# Check that Varnish is alive
+client c1 {
+	txreq
+	rxresp
+} -run