Zero out elements before shrinking the slice to avoid memory leaks.

Fixes #66410

Change-Id: I8f64c21455761f7f7c8b6fee0b6450b98f691d91
GitHub-Last-Rev: b15586e801199b9674f5bfcb12a848a55f15a80b
GitHub-Pull-Request: golang/go#66419
Reviewed-on: https://go-review.googlesource.com/c/go/+/572956


TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>