We would panic when opening a symlink ending in ..,
where the symlink references the root itself.

For #73081
Fixes #73082

Change-Id: I7dc3f041ca79df7942feec58c197fde6881ecae5
Reviewed-on: https://go-review.googlesource.com/c/go/+/661416


Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
(cherry picked from commit cfc784a1)
Reviewed-on: https://go-review.googlesource.com/c/go/+/662315