window: Only draw resize grip if its window exists
window: Only draw resize grip if its window exists

Don't rely on priv->resize_grip_visible as the code comment in the
variable declaration indicates.
This fixes warnings with GtkPlug, which can cause resize_grip_visible to
be TRUE but grid_window to be NULL - running tests/teststatusicon
reproduces this.

This broke with 7ef113ce56a75641175af31267e2075f634267e0

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