This just stops the bleeding (crash) in a special case when screen
number argument of SalFrame::ShowFullScreen is -1 (meaning
'fullscreen spanning all available displays')

It doesn't yet extend the fullscreen window over all screens, this
will be done in a follow-up fix

Change-Id: I2cf48096a1fe1ec33c943f10acb41c59585b325f
Reviewed-on: https://gerrit.libreoffice.org/71965
Tested-by: Jenkins
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>