OutputDevice::getFallbackFont requires mpGraphics to be created/initialized.
Without the check and initialization OutputDevice::getFallbackFont will segfault.

Change-Id: I18225a24a6767933c8f1ff8f1cfd392543a95132
Reviewed-on: https://gerrit.libreoffice.org/14287Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>