aesv8-armx.pl: Avoid buffer overrread in AES-XTS decryption

Original author: Nevine Ebeid (Amazon)
Fixes: CVE-2023-1255

The buffer overread happens on decrypts of 4 mod 5 sizes.
Unless the memory just after the buffer is unmapped this is harmless.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
(Merged from https://github.com/openssl/openssl/pull/20759)
