reflect: panic on Invalid Interface call
reflect: panic on Invalid Interface call

This was initially pushed as part of CL 4876046, found
when logic in exp/template was using the method on
an Invalid value.

R=rsc
CC=golang-dev
http://codereview.appspot.com/4890043

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