io: detect Writers that access io.Copy's buffer after returning

io: detect Writers that access io.Copy's buffer after returning

When the race detector is enabled, scribble over copy buffers with
garbage after Write returns.

For #58452

Change-Id: I25547684bcbef7d302d76736cb02e59c89a640ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/466865
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Damien Neil <dneil@google.com>