CL 47341 added support for decoding non-padded messages. But DecodedLen
still returned a multiple of 5 for messages without a padding, even
though it is possible to calculate the len exactly when using NoPadding.

This change makes DecodedLen return the exact number of bytes that
will be written. A change to the decoding logic is also made so that it
can handle this case.

DecodedLen now has the same behaviour as DecodedLen in encoding/base64.

Fixes #20854

Change-Id: I729e0b1c0946c866fb675c854f835f366dd4b5a4
Reviewed-on: https://go-review.googlesource.com/47710Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>