Assign default storage for vcl_backend_error

Creating "backend synthetic" content was impossible whenever a fetch
had already started, unless storage was assigned (again) explicitly
from VCL.

Fixes #2494

bin/varnishd/cache/cache_fetch.c

@@ -764,6 +764,9 @@ vbf_stp_error(struct worker *wrk, struct busyobj *bo)
 	if (bo->fetch_objcore->stobj->stevedore != NULL)
 		ObjFreeObj(bo->wrk, bo->fetch_objcore);
 
+	if (bo->storage == NULL)
+		bo->storage = STV_next();
+
 	// XXX: reset all beresp flags ?
 
 	HTTP_Setup(bo->beresp, bo->ws, bo->vsl, SLT_BerespMethod);

bin/varnishtest/tests/r02494.vtc

+varnishtest "vcl_back_backend_error default storage"
+
+server s1 {
+	rxreq
+	txresp -nolen -hdr "Content-Length: 10240" -body 1024
+} -start
+
+varnish v1 -vcl+backend {
+	sub vcl_backend_response {
+		set beresp.do_stream = false;
+	}
+	sub vcl_backend_error {
+		set beresp.status = 200;
+		set beresp.ttl = 1h;
+		return (deliver);
+	}
+} -start
+
+client c1 {
+	txreq -url "/"
+	rxresp
+	expect resp.status == 200
+} -run
+client c2 {
+	txreq -url "/"
+	rxresp
+	expect resp.status == 200
+} -run