encoding/json/v2: fix UnmarshalDecode regression with EOF
When EOF is encountered within jsontext.Decoder stream without
starting to parse any token, UnmarshalDecode should report EOF,
rather than converting it into ErrUnexpectedEOF.
This fixes a regression introduced by https://go.dev/cl/689919.
This change only affects code compiled under goexperiment.jsonv2.
Fixes #74835
Change-Id: I7e8e57ab11b462c422c538503ed8c6b91ead53bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/692175
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Sean Liao <sean@liao.dev> Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Jake Bailey <jacob.b.bailey@gmail.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
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