fix: fixes infinit loop in checksum calculation due to integer overflow

[CheerThe2nd]

I noticed that when using nmea::sentence::validate(nmeaBlock); and the passed nmeaBlock contains 3 or more nmea sentences, the checksum generation inside of nmea::sentence::validate halts for ever. After investigating I noticted that, inside of std::string sentence::checksum, a for loop is started which uses a 8 bit unsigned integer as index. And the end condition, for previously named for loop, is bigger than 255. So the for loop can never end since the 8 bit index will keep overflowing back to 0. To Fix this a higher bit value should be used as index.

This snippet will reproduce the issue if used on the old version:

 const std::string testNmea = "$GPGGA,172814.0,3723.46587704,N,12202.26957864,W,2,6,1.2,18.893,M,-25.669,M,2.0,0031*4F\r\n";
    for (int i = 0; i < 15; ++i)
    {
        printf("Now testing block size %d \n", i);
        std::string nmeaBlock;
        for(int k = 0; k < i; ++k)
            nmeaBlock += testNmea;

        printf("Current block %s\n", nmeaBlock.c_str());
        nmea::sentence::validate(nmeaBlock);
    }

    return 0;

[CheerThe2nd]

In hindsight you could also just use the same type, for the index, which is used to store the end condition