XTS bug spotted and fix suggested by Adrian Kotelba.
(Imported from upstream's e620e5ae37bc3fc5e457ebf3edcdd01b20f8c5dd.)
Another patch we missed.
Change-Id: Ibea40eeec01a49b29064b14631706756795c9592
Reviewed-on: https://boringssl-review.googlesource.com/4489
Reviewed-by: Adam Langley <agl@google.com>