This change adds functionality to properly handle NoPadding in NewDecoder.

Removes the following expectations when using NoPadding:

* the input message length is a multiple of 8
* the input message length is 0, or longer than 7 characters

Fixes #25332

Change-Id: I7c38160df23f7e8da4f85a5629530016e7bf71f3
GitHub-Last-Rev: 68ab8d2291df5c69e647620f8ef82cc90e06db28
GitHub-Pull-Request: golang/go#25394
Reviewed-on: https://go-review.googlesource.com/113215
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>