encoding/json: use reflect.Value.UnsafePointer over Pointer

The latter returns a uintptr, while the former returns a unsafe.Pointer.
A uintptr is unsafe if Go ever switches to a moving GC,
while a unsafe.Pointer will be properly tracked by the GC.

We do not use unsafe.Pointer for any unsafe type conversions,
and only use it for comparability purposes, which is relatively safe.

Updates #40592

Change-Id: I813e218668704b63a3043acda4331205a3835a66
Reviewed-on: https://go-review.googlesource.com/c/go/+/360855


Trust: Joseph Tsai <joetsai@digital-static.net>
Trust: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>