encoding/json: fix truncated Token error regression in goexperiment.jsonv2

The jsontext.Decoder.ReadToken method reports a non-EOF error,
if the token stream is truncated and does not form a valid JSON value.
In contrast, the v1 json.Decoder.Token method would report EOF
so long as the input was a prefix of some valid JSON value.
Modify json.Decoder.Token to preserve historical behavior.

This only modifies code that is compiled in under goexperiment.jsonv2.

Updates #69449
Fixes #74750

Change-Id: Ifd281c46f118f0e748076013fefc7659f77c56ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/689516
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

0 files changed, 0 insertions, 0 deletions