cookie: avoid harmless use after free

[pauldreik]

This fix removes a use after free which can be triggered by
the internal cookie fuzzer, but otherwise is probably
impossible to trigger from an ordinary application (Thanks @bagder for checking).

The following program reproduces it:

        curl_global_init(CURL_GLOBAL_DEFAULT);
        CURL*  handle=curl_easy_init();
        CookieInfo* info=Curl_cookie_init(handle,NULL,NULL,false);
        curl_easy_setopt(handle, CURLOPT_COOKIEJAR, "/dev/null");
        Curl_flush_cookies(handle, true);
        Curl_cookie_cleanup(info);
        curl_easy_cleanup(handle);
        curl_global_cleanup();

This was found through fuzzing.

[bagder]

Picky checksrc:

./cookie.c:1649:19: warning: no space after equals sign (EQUALSNOSPACE)
     data->cookies=NULL;
                   ^

[bagder]

Thanks!