runtime: better checks for syscall.NewCallback parameter
runtime: better checks for syscall.NewCallback parameter

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/4859042

wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old