new method to deal with offset==0

new method to deal with offset==0

in this new method, when an `offset==0` is detected,
it's converted into (size_t)(-1), instead of 1.

The logic is that (size_t)(-1) is effectively an extremely large positive number,
which will not pass the offset distance test at next stage (`execSequence()`).
Checked the source code, and offset is always checked (as it should),
using a formula which is not vulnerable to arithmetic overflow:
```
RETURN_ERROR_IF(sequence.offset > (size_t)(oLitEnd - virtualStart),
```

The benefit is that such a case (offset==0) is always detected as corrupted data
as opposed to relying on the checksum to detect the error.