Getting CURLINFO_CONTENT_LENGTH_DOWNLOAD_T during a header call back will return -1.

[Kenji Shukuwa via curl-library]

 Until curl 8.6.0, it was possible to obtain Content-Length using 
CURLINFO_CONTENT_LENGTH_DOWNLOAD_T in the header callback, but after 
https://github.com/curl/curl/pull/13134 was committed, it now returns -1.

 According to the API documentation for curl_easy_getinfo(), "Use this function 
after a performed transfer if you want to get transfer related data.", so it 
may just have been possible to retrieve the data by chance up until now, but is 
the above behavior by design?


Sudo code:
```
static size_t header_callback(char *buffer, size_t size,
                              size_t nitems, void *userdata)
{
  CURL *curl = userdata;
  CURLcode res = curl_easy_getinfo(curl, CURLINFO_CONTENT_LENGTH_DOWNLOAD_T, 
&cl);
  if(!res) {
    printf("Download size: %" CURL_FORMAT_CURL_OFF_T "\n", cl);
  }

  return nitems * size;
}
 
int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com";);

    curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, header_callback);
    curl_easy_setopt(curl, CURLOPT_HEADERDATA, curl);

    curl_easy_perform(curl);
  }
}
```

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