calculate_checksum fails on 64 bit linux ( when long is 8 bytes )

```
There is reading of sizeof(unsigned long) bytes from file, but on 64 bit linux 
long is 8 bytes. When replacing long with any 4 byte size ( like int ) 
everything works fine.
```

Original issue reported on code.google.com by `bog...@gmail.com` on 15 May 2013 at 10:40
