Fix crash if completing with NULL shell_cur
Fix crash if completing with NULL shell_cur

This happens if you do e.g.
  flatpak complete "flatpak install " 11

Reported by aki237

handsome-feng/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,matthiasclasen/flatpak