The Addr might be a stack variable with uninitialized fields.

Fixes #9777.

Change-Id: I799786e3d8b2e17e069725bc66a076cf9ca11f93
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/3932Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Keith Randall <khr@golang.org>