read_null_terminated_utf8_string does not check box bounds

It is possible for a bad source file to read beyond the end of the box returning a bogus string. This will cause error downstream in areas such as in Emsg reader:

let message_size = size - Self::size_without_message(version, &scheme_id_uri, &value);
let mut message_data = Vec::with_capacity(message_size as usize);
reader.read_exact(&mut message_data)?;

The message will not be able to read correctly.