[Headers] segmentation fault on init / add

[susnux]

I have tested this library, but I always got a segmentation fault in:
void curl_header::add(const string header)

After some tests I noticed it is this line:
this->headers = curl_slist_append(this->headers,header.c_str());

I compiled against libcurl 7.38.0 and compiled with no special flags (except maybe -fPIC).
My example to reproduce:

std::initializer_list<std::string> headers = {
    "User-Agent: Mozilla/5.0",
    "Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8",
    "Accept-Encoding: gzip,deflate"};
std::ostringstream resultStream;
curl_writer writer(resultStream);
curl_form form;
curl_easy easy(writer);
curl_header header(headers);
//Never reaches this point```