encoding/base64: make decoder errors independent of read chunking

[larrasket]

The stream decoder decoded each buffered chunk with a stateless call
to Decode, so validity and error offsets depended on how the
underlying reader happened to chunk the input. A padded group followed
by more input was accepted whenever a chunk boundary fell right after
the padding, even though decoding the same input as a whole rejects
it, and CorruptInputError offsets were relative to the current chunk
rather than the whole stream.

Track how much input has been consumed and whether a padded group has
been seen: reject any input that follows a padded group, and rebase
error offsets so they refer to positions in the whole input stream.

Fixes #31626

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Patch Set 1:

(1 comment)

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Message from Salih Gabbir:

Patch Set 2:

(1 comment)

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