rbtree: Don't write to nil node
rbtree: Don't write to nil node

The code used to set nil->parent, which could cause segfaults. Don't do
that. We also need to pass the parent explicitly to the fixup code,
because the node during fixup may be the nil node.

davidt/gtk,jessevdk/gtk,davidgumberg/gtk,Sidnioulz/SandboxGtk,chergert/gtk,alexlarsson/gtk,chergert/gtk,grubersjoe/adwaita,ebassi/gtk,ahodesuka/gtk,alexlarsson/gtk,msteinert/gtk,davidt/gtk,jigpu/gtk,ahodesuka/gtk,davidt/gtk,msteinert/gtk,Adamovskiy/gtk,chergert/gtk,Lyude/gtk-,alexlarsson/gtk,jigpu/gtk,jadahl/gtk,grubersjoe/adwaita,alexlarsson/gtk,Sidnioulz/SandboxGtk,ebassi/gtk,Adamovskiy/gtk,Sidnioulz/SandboxGtk,msteinert/gtk,alexlarsson/gtk,Distrotech/gtk2,davidt/gtk,ahodesuka/gtk,davidgumberg/gtk,jigpu/gtk,ahodesuka/gtk,jadahl/gtk,bratsche/gtk-,davidt/gtk,jadahl/gtk,grubersjoe/adwaita,alexlarsson/gtk,bratsche/gtk-,Lyude/gtk-,ebassi/gtk,davidgumberg/gtk,chergert/gtk,chergert/gtk,jadahl/gtk,grubersjoe/adwaita,chergert/gtk,Distrotech/gtk2,jadahl/gtk,jigpu/gtk,Lyude/gtk-,jigpu/gtk,jadahl/gtk,grubersjoe/adwaita,bratsche/gtk-,ahodesuka/gtk,alexlarsson/gtk,ahodesuka/gtk,ebassi/gtk,jigpu/gtk,alexlarsson/gtk,Adamovskiy/gtk,chergert/gtk,Lyude/gtk-,Sidnioulz/SandboxGtk,jessevdk/gtk,davidgumberg/gtk,chergert/gtk,davidgumberg/gtk,grubersjoe/adwaita,Lyude/gtk-,msteinert/gtk,Lyude/gtk-,davidgumberg/gtk,bratsche/gtk-,jessevdk/gtk,Distrotech/gtk2,Adamovskiy/gtk,ahodesuka/gtk,ahodesuka/gtk,davidt/gtk,Adamovskiy/gtk,jessevdk/gtk,ebassi/gtk,davidgumberg/gtk,Adamovskiy/gtk,jessevdk/gtk,jadahl/gtk,ebassi/gtk,Distrotech/gtk2,Distrotech/gtk2,Sidnioulz/SandboxGtk,davidgumberg/gtk,jadahl/gtk,Lyude/gtk-,grubersjoe/adwaita,jigpu/gtk,msteinert/gtk,Distrotech/gtk2,Adamovskiy/gtk,Lyude/gtk-,msteinert/gtk,bratsche/gtk-,bratsche/gtk-,grubersjoe/adwaita,jigpu/gtk,jessevdk/gtk,Sidnioulz/SandboxGtk,Adamovskiy/gtk,jessevdk/gtk