window: Fix segault on zooming operations
When virt-viewer is "Waiting for guest domain to start" and
the Ctrl- or Ctrl+ keys are pressed to zoom the blank display
virt-viewer will crash in virt_viewer_display_get_desktop_size
because of a NULL display pointer. To reproduce start virt-viewer
on a VM not running and zoom the display.
Signed-off-by: Charles Arnold <carnold@suse.com>
Acked-by: Fabiano FidĂȘncio <fidencio@redhat.com>
Acked-by: Pavel Grunt <pgrunt@redhat.com>