public long readNextLong() throws IOException {
        long l = readLong(currentIndex);
        currentIndex+=SIZEOF_LONG;
        return l;
    }
Reads the long at the current index and then increments the current
index by <code>SIZEOF_LONG</code>.
@return the long at the current index
@exception IOException if an I/O error occurs
GPL/DMG/src/dmg/java/mobiledevices/dmg/ghidra/GBinaryReader.java