reflect: remove a double negative, use the rtype.pointers method for clarity

Change-Id: Ia24e9f0da1622cededa17b2c54ff9e4bb80cf946
Reviewed-on: https://go-review.googlesource.com/1633
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>