sync/atomic: don't atomically write pointers twice

sync/atomic: don't atomically write pointers twice

sync/atomic.StorePointer (which is implemented in
runtime/atomic_pointer.go) writes the pointer twice (through two
completely different code paths, no less). Fix it to only write once.

Change-Id: Id3b2aef9aa9081c2cf096833e001b93d3dd1f5da
Reviewed-on: https://go-review.googlesource.com/21999
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>