Add specs for Rex::ImageSource

And fix reading on ```#read_asciiz```

Verification
----------

- [x] Be sure which travis is green
- [ ] Run the specs locally and ensure all the tests pass

```
$ rspec spec/lib/rex/image_source/*
Rex::ImageSource::Disk ...................
Rex::ImageSource::Memory ..............................

Finished in 0.11164 seconds
49 examples, 0 failures

```


'when offset less than 0'