encoding/gob: support large slices in slice decode helpers
The slice decode helpers weren't aware of partially allocated slices.
Also add large slice support for []byte.
Change-Id: I5044587e917508887c7721f8059d364189831693
Reviewed-on: https://go-review.googlesource.com/c/go/+/443777
Auto-Submit: Ian Lance Taylor <iant@google.com>
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Run-TryBot: Ian Lance Taylor <iant@google.com> Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
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