Miscellany

Alright, this is fixed up. `message_set_int` doesn't need to keep track of how many bytes have been decoded. That's an incidental change, though - the real goal is/was to avoid causing the whole thing to blow up when passed truncated input.

I think it's probably worth making another pass at the whole thing and ensuring that we're not going to crash with `badmatch` on truncated input. This was an easy place to trigger that sort of failure in the existing code, but I suspect there are more. We'll also want to pass `undefined` values back up the stack. For example, `kofta_fetch:decode/1` needs to handle `undefined` results from `kofta_decode:message_set/1`.