Stop incorrectly reporting overflow on zeroed ws.

Ref: #2008

bin/varnishd/cache/cache_panic.c

@@ -116,7 +116,7 @@ pan_ws(struct vsb *vsb, const struct ws *ws)
 	VSB_printf(vsb, "ws = %p {\n", ws);
 	VSB_indent(vsb, 2);
 	PAN_CheckMagic(vsb, ws, WS_MAGIC);
-	if (!(ws->id[0] & 0x20))
+	if (ws->id[0] != '\0' && (!(ws->id[0] & 0x20)))
 		VSB_printf(vsb, "OVERFLOWED ");
 	VSB_printf(vsb, "id = \"%s\",\n",  ws->id);
 	VSB_printf(vsb, "{s,f,r,e} = {%p",  ws->s);

bin/varnishd/cache/cache_ws.c

@@ -254,6 +254,7 @@ int
 WS_Overflowed(const struct ws *ws)
 {
 	CHECK_OBJ_NOTNULL(ws, WS_MAGIC);
+	AN(ws->id[0]);
 
 	if (ws->id[0] & 0x20)
 		return (0);