Previously we allowed drivers to modify the row buffer used to scan
values when closing Rows. This is no longer acceptable and can lead
to data races.

Fixes #23519

Change-Id: I91820a6266ffe52f95f40bb47307d375727715af
Reviewed-on: https://go-review.googlesource.com/89936
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>