avro: Unmarshal should fail on extra data

[rogpeppe]

If there's more data in the buffer than needed, Unmarshal will succeed, but it should probably fail.

[sixstone-qq]

@rogpeppe could you specify better the context of the issue and reopen it in this case :pray: ? Thanks a lot!

[rogpeppe]

Here's an example of when if succeeds but should probably fail: https://play.golang.org/p/T2Uict2TvqE

I can't reopen the issue because I don't have write access to the repo :)