Fix bug #67704: CURLOPT_POSTFIELDS modify the type of param

curl_setopt will modify the type of param
the bugs is here: https://bugs.php.net/bug.php?id=67704

when the type of param is not string, we use a temp variable(current_tmp in the code)， then convert it to string, free the memory of the variable at last
You can use a normal `zval` instead of `zval *` to reduce memory allocation.