hw/uefi: check auth.hdr_length minimum size

auth.hdr_length maximum is already checked (against buffer size).  The
header has some fixed fields which are included in the header length, so
there also is a minimum size which must be verified.  Add a check for
that.  Fixes possible integer underflow.

While being at it replace the magic number '24' with sizeof calculations
for better code documentation.

Fixes: CVE-2026-8341
Fixes: f1488fac0584 ("hw/uefi: add var-service-auth.c")
Reported-by: Feifan Qian <bea1e@proton.me>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-ID: <20260512060523.17493-1-kraxel@redhat.com>