#vulnerable code 
                                public int byteCheckSum() throws IORuntimeException {
        if (readLimit() >= Integer.MAX_VALUE || start() != 0)
            return super.byteCheckSum();
        byte b = 0;
        NativeBytesStore bytesStore = (NativeBytesStore) bytesStore();
        for (int i = (int) readPosition(), lim = (int) readLimit(); i < lim; i++) {
            b += bytesStore.memory.readByte(bytesStore.address + i);
        }
        return b & 0xFF;
    } 
                            #location 7 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                public int byteCheckSum() throws IORuntimeException {
        if (readLimit() >= Integer.MAX_VALUE || start() != 0)
            return super.byteCheckSum();
        byte b = 0;
        NativeBytesStore bytesStore = (NativeBytesStore) bytesStore();
        Memory memory = bytesStore.memory;
        assert memory != null;
        for (int i = (int) readPosition(), lim = (int) readLimit(); i < lim; i++) {
            b += memory.readByte(bytesStore.address + i);
        }
        return b & 0xFF;
    }