encoding/base32: make NoPadding Encoding's DecodedLen return exact size
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.