Set element in slice to nil avoiding memory leak.

Change-Id: I9dbef9a0466407011e326725d3a0b681cd815389
GitHub-Last-Rev: 1bae5d375876a7f146eb04c2a5ba88d079264eb3
GitHub-Pull-Request: golang/go#30386
Reviewed-on: https://go-review.googlesource.com/c/163601
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>