archive/zip: fix panic in Reader.Open
When operating on a Zip file that contains a file prefixed with "../",
Open(...) would cause a panic in toValidName when attempting to strip
the prefixed path components.
Fixes CVE-2021-27919
Fixes #44916
Change-Id: Ic755d8126cb0897e2cbbdacf572439c38dde7b35
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/
1004761
Reviewed-by: Filippo Valsorda <valsorda@google.com>
Reviewed-by: Russ Cox <rsc@google.com>
Reviewed-by: Katie Hockman <katiehockman@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/300489
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>