The cookie flushing (saving to a cookie jar) should only be done if a
transfer has been started. This is now done by checking the
cookies->running field, which is not reset in curl_easy_reset() so the
saving works correctly even after a call to that.
Follow-up to fd6eb8d6e7
Verified by test 1920
Reported-by: Alexander Batischev
Fixes#20090Closes#20094
