reader: give slice a larger capacity than size

This causes Validate to fail if someone returns
cap(b) instead of len(b) in the Read method.

Fixes #golang/go#11360

Change-Id: Ic6dcd60df842288ddeaaf6e212cc7693cb9b0257
Reviewed-on: https://go-review.googlesource.com/15909
Reviewed-by: Andrew Gerrand <adg@golang.org>
1 file changed