Fixes #25435

The added test fails without the re-slice and passes with it.

Change-Id: I5ebc2a737285eb116ecc5938d8bf49050652830f
GitHub-Last-Rev: 454ddad7df8d56a1d0e05a999ed8277c5516ce01
GitHub-Pull-Request: golang/go#25436
Reviewed-on: https://go-review.googlesource.com/113495Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>