encoding/json: always verify we can get a field's value

Calling .Interface on a struct field's reflect.Value isn't always safe.
For example, if that field is an unexported anonymous struct.

We only descended into this branch if the struct type had any methods,
so this bug had gone unnoticed for a few release cycles.

Add the check, and add a simple test case.

Fixes #28145.

Change-Id: I02f7e0ab9a4a0c18a5e2164211922fe9c3d30f64
Reviewed-on: https://go-review.googlesource.com/c/141537


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>