[release-branch.go1.8] reflect: clear ptrToThis in Ptr when allocating result on heap

Otherwise, calling PtrTo on the result will fail.

Fixes #19003

Change-Id: I8d7d1981a5d0417d5aee52740469d71e90734963
Reviewed-on: https://go-review.googlesource.com/36731


Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-on: https://go-review.googlesource.com/36718


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>