cas: clean blockTransform, panic intentionally

There was a bug in diskv (https://github.com/peterbourgon/diskv/pull/20) which
meant that the blockTransform function could be passed an empty string. In our
case this just paniced after trying to split + access parts of the resulting
empty slice.

Panicking is arguably correct behaviour (if we ever get something that's not a
hash it's likely a bug in our code), but let's be more intentional about it.