Currently the only non-ZIP-based import filter that declares the
SUPPORTSSIGNING flag is PDF, so if we get a stream without a storage, we
assume it's PDF.

If any other non-ZIP-based format would add that flag in the future,
that would mean PDFDocument::Read() gets that as an input. That means it
makes sense to at least check the file header early in the tokenizer,
and return early when that doesn't match.

Change-Id: I8760d130c4211f37be705e03b22814825042cac8
Reviewed-on: https://gerrit.libreoffice.org/29888Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>