Nested messages, subtraction with oveflow panic

[koivunej]

I am not sure how to fix this. Here's a reproducer that works on top of 75a0517 in quick-protobuf/src/reader.rs.

#[test]
fn read_foomessage_panic() {
    let bytes = &[0, 93, 138, 1, 1, 170, 1, 0];
    let mut r = BytesReader::from_bytes(bytes);

    assert!(!r.is_eof());
    assert_eq!(r.next_tag(bytes).unwrap(), 0);
    r.read_unknown(bytes, 0).unwrap();

    assert!(!r.is_eof());
    assert_eq!(r.next_tag(bytes).unwrap(), 138);
    r.read_message::<Example>(bytes).unwrap();

    struct Example;

    impl<'a> MessageRead<'a> for Example {
        fn from_reader(r: &mut BytesReader, bytes: &'a [u8]) -> Result<Self> {
            assert!(!r.is_eof());
            assert_eq!(r.next_tag(bytes).unwrap(), 170);

            // there's nothing to read for the nested message
            assert_eq!(r.len(), 0);

            // this will call BytesReader::len which will trigger the subtraction with overflow
            r.read_packed_fixed::<&[f32]>(bytes).unwrap();
            unreachable!()
        }
    }
}

Found with #210.