wibox.widget.textbox: return 0, 0 from fit() if either w or h is 0

A hack around abusing the fact that width of a textbox is 0 when its
empty, while it's height is still set according to the font.

Signed-off-by: Lukáš Hrázký <lukkash@email.cz>
Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/wibox/widget/textbox.lua.in

@@ -45,6 +45,11 @@ end
 function textbox:fit(width, height)
     setup_layout(self, width, height)
     local ink, logical = self._layout:get_pixel_extents()
+
+    if logical.width == 0 or logical.height == 0 then
+        return 0, 0
+    end
+
     return logical.width, logical.height
 end