Extend the fix for issue #12 to handle much larger files

The fix for issue #12 (closed) enables conversion of 16G and 32G files, but any file greater than 32G will have the same problem as issue #12 (closed). Though a file greater than 32G are extremely large, we will likely reach that limit at some point. The fix seems to be simple, use type size_t rather than type unsigned. I have no idea what this might do to performance.