Correctly unref widget_nodes

Correctly unref widget_nodes

While drawing the wibox, the C core builds up a list of widgets and their
associated geometry. This list consists of widget_node_t objects and is
constructed like this (This is from luaA_table2widgets()):

   widget_node_t w;
   p_clear(&w, 1);
   w.widget = luaA_object_ref_item(L, idx, -1);
   widget_node_array_append(widgets, w);

After we are done with this list of widget nodes, it is freed via
wibox_widget_node_array_wipe(). However, wibox_widget_node_array_wipe() passed
"&w" to luaA_object_unref_item() while it should have used "w.widget" (which is
what was returned from luaA_object_ref_item()). This made the unref fail and the
wibox was carrying around a reference to this widget forever. This was
effectively a memory leak.

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