This caused not finding the length of a stream -> could not actually
verify signature.

Change-Id: I696b6da49525eb53f7575c27f619d2116be51f1d
Reviewed-on: https://gerrit.libreoffice.org/31490Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>