Make readHeader check the header size before using it.

        final HashMap<String, String> attributes = new HashMap<String, String>();

        final int headerSize;

        headerSize = buffer.readInt();

        populateOptions(buffer, headerSize, attributes);

        buffer.position(headerSize);

        if (headerSize < 0) {

            throw new UnsupportedFormatException("header size can't be negative.");

        }

        populateOptions(buffer, headerSize, attributes);

        buffer.position(headerSize);

        final FileHeader header = new FileHeader(headerSize,

                new FusionDictionary.DictionaryOptions(attributes,

                        0 != (optionsFlags & FormatSpec.GERMAN_UMLAUT_PROCESSING_FLAG),