Bug#788704: gnutls28: VIA PadLock accelerated AES-CBC segfaults

[Peter Lebbing]

As indicated by Nikos Mavrogiannopoulos on the gnutls-devel mailing list[1],
this problem had been fixed upstream in 3.3.12.

I had completely forgotten to check upstream for fixes.

My suggested patch is almost exactly the same as commit 023156a from the GnuTLS
Git[2].

I'd like to suggest backporting that commit to GnuTLS in jessie/stable, to not
deviate unnecessarily from upstream and fix the problem.

The message [1] also mentions a second commit which prevents calling the code
with a length 0 in the first place, as it is a useless action. That commit is
not necessary to fix this specific bug.

With regards,

Peter.

[1] https://lists.gnupg.org/pipermail/gnutls-devel/2015-June/007627.html

[2] https://gitlab.com/gnutls/gnutls/commit/023156ae2504c1911f8f2e66a0ebde316931671c

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