VRT_TIME_string to trigger VCL failure for unformatable time

as per pow-wow decision

Ref #3308

bin/varnishd/cache/cache_vrt.c

@@ -837,17 +837,17 @@ VCL_STRING v_matchproto_()
 VRT_TIME_string(VRT_CTX, VCL_TIME t)
 {
 	char *p;
-	uintptr_t snapshot;
 
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
-	snapshot = WS_Snapshot(ctx->ws);
 	p = WS_Alloc(ctx->ws, VTIM_FORMAT_SIZE);
-	if (p != NULL) {
-		VTIM_format(t, p);
-		if (*p == '\0') {
-			p = NULL;
-			WS_Reset(ctx->ws, snapshot);
-		}
+	if (p == NULL) {
+		VRT_fail(ctx, "Workspace overflow");
+		return (NULL);
+	}
+	VTIM_format(t, p);
+	if (*p == '\0') {
+		VRT_fail(ctx, "Unformatable VCL_TIME");
+		return (NULL);
 	}
 	return (p);
 }

bin/varnishtest/tests/r03308.vtc

@@ -16,5 +16,5 @@ varnish v1 -vcl+backend {
 client c1 {
 	txreq
 	rxresp
-	expect resp.http.ts == ""
+	expect resp.status == 503
 } -run