encoding/gob: make encoding structs a little faster
FieldByIndex never returns an invalid Value, so the validity
test can be avoided if the field is not indirect.

BenchmarkGobEncode     12768642      12424022      -2.70%
BenchmarkGobEncode     60.11        61.78        1.03x

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/158890045