For ODF signatures we require that all streams of the storage are
signed.  The PDF equivalent of this is to ensure that the byte range is
the entire file, including the signature dictionary but excluding the
signature value itself.

Change-Id: Ie47f42913e2aa960f35079eb981768cd47fb9f92
Reviewed-on: https://gerrit.libreoffice.org/29890Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>