encoding/json: fix extra data regression under goexperiment.jsonv2
When operating under v1 semantics in the v2 implementation,
a extra data error should take precedence over any semantic error
that could theoretically occur within the value itself.
This change only affects code compiled under goexperiment.jsonv2.
Fixes #74614
Change-Id: I055a606b053fa66b0c766ae205487b8290109285
Reviewed-on: https://go-review.googlesource.com/c/go/+/689919 Reviewed-by: Damien Neil <dneil@google.com> Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
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