xdg_shell stable: fix zero-sized positioner anchor_rect
xdg_shell stable: fix zero-sized positioner anchor_rect

We used 0 as unset-check value before, which was fine when 0-width
was invalid, but isn't anymore

ascent12/wlroots,SirCmpwn/wlroots,swaywm/wlroots,ascent12/wlroots,swaywm/wlroots