static void free_fixed_hostname(struct hostname *host)
{
#if defined(USE_LIBIDN2)
  if(host->encalloc) {
    idn2_free(host->encalloc); /* must be freed with idn2_free() since this was
                                 allocated by libidn */
    host->encalloc = NULL;
  }
#elif defined(USE_WIN32_IDN)
  free(host->encalloc); /* must be freed with free() since this was
                           allocated by curl_win32_idn_to_ascii */
  host->encalloc = NULL;
#else
  (void)host;
#endif
}

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.