Call check_widget() in :setup()

I just spent too much time tracking down a bug that happened while drawing a
widget. This is the reason why we should apply sanity checks while widgets are
constructed, so that we get a useful backtrace.

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

lib/wibox/widget/base.lua

@@ -390,6 +390,7 @@ local function drill(ids, content)
                 e, id2 = drill(ids, v)
                 widgets[k] = e
             end
+            base.check_widget(widgets[k])
 
             -- Place the widget in the access table
             if id2 then