So that when we have a single signature with ID="Signature2", then we
use "Signature3" for the next ID, not "Signature2". (Acrobat uses that ID
for the first signature.)

Change-Id: I7032fbf014184da2a5be24730a92abc32a9a1258
Reviewed-on: https://gerrit.libreoffice.org/30725Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>