text/template: fix slice builtin for pointers to arrays

text/template: fix slice builtin for pointers to arrays

The slice function now properly handles pointers to arrays by calling
indirect() to dereference pointers, matching the behavior of the index
function.

Fixes #39596

Change-Id: Id4920edbfd8fd3df3a181b59a61733f88b0f104d
Reviewed-on: https://go-review.googlesource.com/c/go/+/727400
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>