The following code appears to fix it but I'm not 100% sure if that's correct. In particular I'm not really sure where the RTP output code is set up.

  curl_write_callback writeit;
  void *user_ptr;

  if(len == 0) {
    failf(data, "Cannot write a 0 size RTP packet.");
    return CURLE_WRITE_ERROR;
  }

  if(data->set.fwrite_rtp) {
    writeit = data->set.fwrite_rtp;
    user_ptr = data->set.rtp_out;
  }
  else
  {
    writeit = data->set.fwrite_func;
    user_ptr = data->set.out;
  }

  if(!data->set.fwrite_rtp && !data->set.is_fwrite_set &&
     user_ptr == NULL) {
    /* if no callback is set for either RTP or default, the default function
       fwrite() is utilized and that can't handle a NULL input */
    failf(data, "No destination to default data callback!");
    return CURLE_WRITE_ERROR;
  }

  wrote = writeit(ptr, 1, len, user_ptr);

I did my change like that since I was convinced that CURLOPT_INTERLEAVEDATA was documented to get passed to the WRITEFUNCTION if no INTERLEAVEFUNCTION was set, but now when I read the docs again with an extra eye on this I can see no hint or indication that it would actually work like this.

But still the code does make this decision, so the question is then who's right. The code or the (lack of) docs saying it should work like that.

I think I'm inclined to rule against the code here and in favor of the documentation. So yes, let's pass the data to the regular callback + writedata unless the INTERLEAVEFUNCTION is set and then use both. The code can be simplified a bit then since it won't send a NULL to fwrite() using the default setup, so the entire extra precaution I added can be removed again:

  curl_write_callback writeit;
  void *user_ptr;

  if(len == 0) {
    failf(data, "Cannot write a 0 size RTP packet.");
    return CURLE_WRITE_ERROR;
  }

  if(data->set.fwrite_rtp) {
    writeit = data->set.fwrite_rtp;
    user_ptr = data->set.rtp_out;
  }
  else
  {
    writeit = data->set.fwrite_func;
    user_ptr = data->set.out;
  }

  wrote = writeit(ptr, 1, len, user_ptr);

Can you perhaps amend this by also making some clarification in the documentation?