The maximum input plaintext for GCM is 64GiB - 64. Since the GCM
interface is one-shot, it's very hard to hit this in Go (one would need
a 64GiB buffer in memory), but we should still enforce this limit.

Thanks to Quan Nguyen for pointing it out.

Change-Id: Icced47bf8d4d5dfbefa165cf13e893205c9577b8
Reviewed-on: https://go-review.googlesource.com/28410
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>