If the style does not actually have any font items, as is the case for
frame, page and number styles in Writer, the maFont will be
default-initialized and the maPixelSize incorrect.
Avoid using these variables.

(cherry picked from commit 07df816d)

Change-Id: I084cd8faa90a3d53310ceec55e8dae3af3dd586c
Reviewed-on: https://gerrit.libreoffice.org/18383Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>