ARG_CLEAR credential scrubbing wipes only UTF-8 copies on Windows Unicode builds

`cleanarg()` operates on `char *str`, which is the UTF-8 copy allocated by `convert_tchar_to_UTF8()`. It has no reference to the original `wchar_t argv[]` element.

/* tool_getparam.c:632 */
static void cleanarg(char *str)
{
  /* now that getstr has copied the contents of nextarg, wipe the next
   * argument out so that the username:password is not displayed in the
   * system process list */
  if(str) {
    size_t len = strlen(str);
    memset(str, '*', len); …