cookies: ensure that we have cookies before writing jar

The jar should be written iff there are cookies, so ensure that we still
have cookies after expiration to avoid creating an empty file.

Closes #2529

lib/cookie.c

@@ -1442,6 +1442,10 @@ static int cookie_output(struct CookieInfo *c, const char *dumphere)
   /* at first, remove expired cookies */
   remove_expired(c);
 
+  /* make sure we still have cookies after expiration */
+  if(0 == c->numcookies)
+    return 0;
+
   if(!strcmp("-", dumphere)) {
     /* use stdout */
     out = stdout;