How to prevent libcurl(c++) form downloading binary data?

Question

I am making a web crawler and I have the following code but the problem is that it also downloads binary data and I don't want that to happen. How do I prevent it

size_t HTML::WriteMemoryCallback(void *contents, size_t size, size_t nmemb, void *userp) {
    size_t realsize = size * nmemb;
    if(contents!=NULL||userp!=NULL){
        std::string* str=(std::string*)userp;
        str->reserve(realsize);
        auto c_str=(char*)contents;
        if(c_str!=NULL){
            for(size_t i=0;i<realsize;i++){
                str->push_back(c_str[i]);
            }
        }
    }
    return realsize;
}

HTML_CODE HTML::get_html(std::string url) {
    std::string chunk;
    CURL *curl_handle=curl_easy_init();
    CURLcode res;
    if(curl_handle) {
        curl_easy_setopt(curl_handle, CURLOPT_URL, url.c_str());
        curl_easy_setopt(curl_handle, CURLOPT_FOLLOWLOCATION, 1L);
        curl_easy_setopt(curl_handle, CURLOPT_WRITEFUNCTION, WriteMemoryCallback);
        curl_easy_setopt(curl_handle, CURLOPT_WRITEDATA, (void *)&chunk);
        curl_easy_setopt(curl_handle, CURLOPT_USERAGENT, USER_AGENT);

        res = curl_easy_perform(curl_handle);

        if(res != CURLE_OK) {
            std::cout<<"Can't get html content from "<<url<<"\n";
            fprintf(stderr, "error: %s\n", curl_easy_strerror(res));
            return {"",""};
        }
        curl_easy_cleanup(curl_handle);
    }
    else{
        std::cout<<"Error: Couldn't create a curl instance"<<std::endl;
        return {"",""};
    }
    return {.url=url,.content=chunk};
}

Things I have tried:-

1. Check if the data has a null terminator
2. Check if the char is an assci letter(it wont work with other language)