encoding/json: add a Fuzz function

encoding/json: add a Fuzz function

Adds a sample Fuzz test function to package encoding/json following the
guidelines defined in #31309, based on
https://github.com/dvyukov/go-fuzz-corpus/blob/master/json/json.go

Fixes #31309
Updates #19109

Change-Id: I5fe04d9a5f41c0de339f8518dae30896ec14e356
Reviewed-on: https://go-review.googlesource.com/c/go/+/174058
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>