Mark a bug in awful.widget.common as such

No idea what the correct value for this argument is supposed to be, but since
there is no variable "m" in scope, this always uses nil as the value.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/widget/common.lua

@@ -95,7 +95,8 @@ function common.list_update(w, buttons, label, data, objects)
         end
         bgb:set_bg(bg)
         if type(bg_image) == "function" then
-            bg_image = bg_image(tb,o,m,objects,i)
+            -- TODO: Why does this pass nil as an argument?
+            bg_image = bg_image(tb,o,nil,objects,i)
         end
         bgb:set_bgimage(bg_image)
         if icon then