listbox: fix a segfault
listbox: fix a segfault

Don't dereference a NULL pointer.

grubersjoe/adwaita,chergert/gtk,davidgumberg/gtk,davidgumberg/gtk,chergert/gtk,jessevdk/gtk,grubersjoe/adwaita,Lyude/gtk-,davidgumberg/gtk,davidgumberg/gtk,Adamovskiy/gtk,ahodesuka/gtk,grubersjoe/adwaita,jigpu/gtk,jigpu/gtk,Lyude/gtk-,alexlarsson/gtk,jadahl/gtk,ahodesuka/gtk,Lyude/gtk-,jigpu/gtk,Adamovskiy/gtk,Sidnioulz/SandboxGtk,jadahl/gtk,ahodesuka/gtk,jessevdk/gtk,davidgumberg/gtk,Adamovskiy/gtk,Lyude/gtk-,Sidnioulz/SandboxGtk,jigpu/gtk,jessevdk/gtk,Lyude/gtk-,alexlarsson/gtk,Sidnioulz/SandboxGtk,Sidnioulz/SandboxGtk,Sidnioulz/SandboxGtk,jadahl/gtk,chergert/gtk,jadahl/gtk,ahodesuka/gtk,ahodesuka/gtk,davidgumberg/gtk,jigpu/gtk,alexlarsson/gtk,alexlarsson/gtk,chergert/gtk,grubersjoe/adwaita,grubersjoe/adwaita,davidgumberg/gtk,davidgumberg/gtk,ahodesuka/gtk,Adamovskiy/gtk,Adamovskiy/gtk,jessevdk/gtk,jadahl/gtk,alexlarsson/gtk,Adamovskiy/gtk,jigpu/gtk,ahodesuka/gtk,Sidnioulz/SandboxGtk,chergert/gtk,jadahl/gtk,jadahl/gtk,ahodesuka/gtk,chergert/gtk,grubersjoe/adwaita,Lyude/gtk-,Adamovskiy/gtk,alexlarsson/gtk,Adamovskiy/gtk,jessevdk/gtk,grubersjoe/adwaita,chergert/gtk,jigpu/gtk,grubersjoe/adwaita,Lyude/gtk-,Lyude/gtk-,jadahl/gtk,jessevdk/gtk,chergert/gtk,alexlarsson/gtk,jigpu/gtk,jessevdk/gtk,alexlarsson/gtk