log/slog: ensure ReplaceAttr does not see a group

log/slog: ensure ReplaceAttr does not see a group

The ReplaceAttr function should not see groups, only leaf attributes.

Previously, we checked an Value for being a group, then resolved it,
then called ReplaceAttr. We neglected to see if it was a group
after resolving it.

Now we resolve first, then check.

Fixes #62731.

Change-Id: I2fc40758e77c445f82deb2c9de8cae7a3b0e22cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/530478
Reviewed-by: Alan Donovan <adonovan@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>