os: avoid finalizer race in windows process object

If proc.Release is called concurrently, a handle will be double-freed.

Change-Id: I0c0c32e312e07bc8615e0bf9e9b691214444d8d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/322510


Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>