rbtree: Replace nil node allocation with a preallocated nil
rbtree: Replace nil node allocation with a preallocated nil

This has one major caveat: The new value is const, so read-only memory.
Any attempt to write to it will cause a crash. Note that we are not ever
supposed to write to it, but bugs happen...

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