private <T> T mergeObjectEncodedAsGroup(T value, final Schema<T> schema) throws IOException
    {
        if (value == null)
            value = schema.newMessage();
        schema.mergeFrom(this, value);
        if (!schema.isInitialized(value))
            throw new UninitializedMessageException(value, schema);
        // handling is in #readFieldNumber
        checkLastTagWas(0);
        return value;
    }
Read an {@code sint64} field value from the internal buffer.
protostuff-core/src/main/java/io/protostuff/ByteArrayInput.java