[release-branch.go1.4] runtime: don't return a slice with nil ptr but non-zero len from growslice

Fixes #10135.

Change-Id: Ic4c5ab15bcb7b9c3fcc685a788d3b59c60c26e1e
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/7400


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-on: https://go-review.googlesource.com/14248


Reviewed-by: Andrew Gerrand <adg@golang.org>