xts: fix incorrect length check

This change does two things:
 1. Fix a length checking bug in the Decrypt function.
 2. Use binary.LittleEndian for byte conversions.

Fixes golang/go#19881

Change-Id: I9d33b92f2bd7e6ca8f69308388f1e8a5c6df2be8
Reviewed-on: https://go-review.googlesource.com/39954
Reviewed-by: Adam Langley <agl@golang.org>
2 files changed