cmd/link: fix invalid usage of reflect.SliceHeader

Caught by "go vet" built with golang.org/cl/248192.

Change-Id: I446083533dd82ecef8db591beb7bd3d70b040d4a
Reviewed-on: https://go-review.googlesource.com/c/go/+/268099


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>