failf: use private buffer, don't clobber receive buffer

{

  va_list ap;

  size_t len;

  char error[CURL_ERROR_SIZE + 2];

  va_start(ap, fmt);

  vsnprintf(data->state.buffer, BUFSIZE, fmt, ap);

  vsnprintf(error, CURL_ERROR_SIZE, fmt, ap);

  len = strlen(error);

  if(data->set.errorbuffer && !data->state.errorbuf) {

    snprintf(data->set.errorbuffer, CURL_ERROR_SIZE, "%s", data->state.buffer);

    strcpy(data->set.errorbuffer, error);

    data->state.errorbuf = TRUE; /* wrote error string */

  }

  if(data->set.verbose) {

    len = strlen(data->state.buffer);

    if(len < BUFSIZE - 1) {

      data->state.buffer[len] = '\n';

      data->state.buffer[++len] = '\0';

    }

    Curl_debug(data, CURLINFO_TEXT, data->state.buffer, len, NULL);

    error[len] = '\n';

    error[++len] = '\0';

    Curl_debug(data, CURLINFO_TEXT, error, len, NULL);

  }

  va_end(ap);