reader: move error check in validate

The method can report the error and return
a bit earlier if there is a problem.

Change-Id: Icf6c258cb0f994cebc7a717e1e28e7b6ec95dd60
Reviewed-on: https://go-review.googlesource.com/15908
Reviewed-by: Andrew Gerrand <adg@golang.org>
1 file changed