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

gnanderson/go,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,gnanderson/go,dsriram/golang,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,dsriram/golang,gnanderson/go,tv42/old-go,tv42/old-go,gnanderson/go,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,tv42/old-go,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,gnanderson/go