paned: Fix size allocation
paned: Fix size allocation

We were passing negative widths to height-for-width calls that
didn't expect this.

grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita