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
https://golang.org/cl/4890043

src/pkg/reflect/value.go

@@ -860,6 +860,9 @@ func (v Value) Interface() interface{} {
 }
 
 func (iv internalValue) Interface() interface{} {
+	if iv.kind == 0 {
+		panic(&ValueError{"reflect.Value.Interface", iv.kind})
+	}
 	if iv.method {
 		panic("reflect.Value.Interface: cannot create interface value for method with bound receiver")
 	}