This changes decoder.Read to always return io.ErrUnexpectedEOF if the input
contains surplus padding or unexpected content. Previously the error could
be io.EOF or io.ErrUnexpectedEOF depending on how the input was chunked.

Fixes #25296

Change-Id: I07c36c35e6c83e795c3991bfe45647a35aa58aa4
GitHub-Last-Rev: 818dfda90b0edf9fc415da4579c5810268c1cdba
GitHub-Pull-Request: golang/go#25319
Reviewed-on: https://go-review.googlesource.com/112516
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>