This allows gins to let Naddr fill in p.From and p.To directly,
avoiding the zeroing and copying of a temporary.

Change-Id: I96d120afe266e68f94d5e82b00886bf6bd458f85
Reviewed-on: https://go-review.googlesource.com/7742Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>