rhbz#1079672: FileDialogHelper: don't hand out stale preview Graphic

The maGraphic is updated from a timer, and if you double-click on a file
then the dialog may exit with the previous file still previewed in
maGraphic, so the wrong image is returned by the dialog.

Change-Id: I99094d85d8d68d5c8a842f52e7039dbbbf095995
(cherry picked from commit f1589f768bbd1d8baea6e442f392831851bdbb3b)
Reviewed-on: https://gerrit.libreoffice.org/10804
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>