... in case NO_PROXY takes an effect

Without this patch, the following command crashes:

    $ GIT_CURL_VERBOSE=1 NO_PROXY=github.com HTTPS_PROXY=https://example.com \
        git clone https://github.com/curl/curl.git

Minimal libcurl-based reproducer:

    #include <curl/curl.h>

    int main() {
      CURL *curl = curl_easy_init();
      if(curl) {
        CURLcode ret;
        curl_easy_setopt(curl, CURLOPT_URL, "https://github.com/");
        curl_easy_setopt(curl, CURLOPT_PROXY, "example.com");
        /* set the proxy type */
        curl_easy_setopt(curl, CURLOPT_PROXYTYPE, CURLPROXY_HTTPS);
        curl_easy_setopt(curl, CURLOPT_NOPROXY, "github.com");
        curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
        ret = curl_easy_perform(curl);
        curl_easy_cleanup(curl);
        return ret;
      }
      return -1;
    }

Assisted-by: Kamil Dudka
Bug: https://bugzilla.redhat.com/1873327