emit error on invalid header

This emits an error when the checksum is incorrect (i.e. if you try to extract a .tgz without unzipping it first)

(I guessed the header is null where the checksum is the initial value, because there I didn't want to harm perf yet, and to make minimal changes)

I can add some test coverage if you want, this is a sort of PITA that wants a lot of coverage.