encoding/json: avoid assuming side-effect free reflect.Value.Addr().Elem()

encoding/json: avoid assuming side-effect free reflect.Value.Addr().Elem()

Consider the following:
	type child struct{ Field string }
	type parent struct{ child }

	p := new(parent)
	v := reflect.ValueOf(p).Elem().Field(0)
	v.Field(0).SetString("hello")           // v.Field = "hello"
	v = v.Addr().Elem()                     // v = *(&v)
	v.Field(0).SetString("goodbye")         // v.Field = "goodbye"

It would appear that v.Addr().Elem() should have the same value, and
that it would be safe to set "goodbye".
However, after CL 66331, any interspersed calls between Field calls
causes the RO flag to be set.
Thus, setting to "goodbye" actually causes a panic.

That CL affects decodeState.indirect which assumes that back-to-back
Value.Addr().Elem() is side-effect free. We fix that logic to keep
track of the Addr() and Elem() calls and set v back to the original
after a full round-trip has occured.

Fixes #24152
Updates #24153

Change-Id: Ie50f8fe963f00cef8515d89d1d5cbc43b76d9f9c
Reviewed-on: https://go-review.googlesource.com/97796
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>