Protobuf parser state handling wrong for implicit close (END_OBJECT)

[cowtowncoder]

When ProtobufParser reaches end of input at top level, it will both:

1. Report JsonToken.END_OBJECT to close root level implicit Object value
2. Call JsonParser.close() to close input

While this works in many ways (current token properly set; further JsonParser.nextToken() calls will return null), it will not work wrt FasterXML/jackson-core#1438 .
Ideally we would probably add one more state (STATE_ROOT_END?) to cover this case.

[cowtowncoder]

Places to change in ProtobufParser:

    @Override
    public JsonToken nextToken() throws IOException {
        ...
// 591:
        case STATE_ROOT_KEY:
            // end-of-input?
            if (_inputPtr >= _inputEnd) {
                if (!loadMore()) {
                    close();
                    return _updateToken(JsonToken.END_OBJECT);
                }
           ....
// 689:
        case STATE_MESSAGE_END: // occurs if we end with array
            close(); // sets state to STATE_CLOSED
            return _updateToken(JsonToken.END_OBJECT);
         ...
    }

    private JsonToken _skipUnknownField(int tag, int wireType) throws IOException
    {
// 961:
            if (_state == STATE_NESTED_KEY) {
                if (_inputPtr >= _inputEnd) {
                    loadMoreGuaranteed();
                }
            } else if (_inputPtr >= _inputEnd) {
                if (!loadMore()) {
                    close();
                    return _updateToken(JsonToken.END_OBJECT);
                }
            }

// 1022
    @Override
    public boolean nextFieldName(SerializableString sstr) throws IOException
    {
        if (_state == STATE_ROOT_KEY) {
            if (_inputPtr >= _inputEnd) {
                if (!loadMore()) {
                    close();
                    _updateToken(JsonToken.END_OBJECT);
                    return false;
                }
            }
// 1103:
    @Override
    public String nextFieldName() throws IOException
    {
        if (_state == STATE_ROOT_KEY) {
            if (_inputPtr >= _inputEnd) {
                if (!loadMore()) {
                    close();
                    _updateToken(JsonToken.END_OBJECT);
                    return null;
                }
            }