gtktextlayout: fix to quiet a warning

Gcc complains about a possible use of a not initialised
widget ref ( which can't happen in reality )

https://bugzilla.gnome.org/show_bug.cgi?id=741702

gtk/gtktextlayout.c

@@ -1640,7 +1640,7 @@ add_child_attrs (GtkTextLayout      *layout,
   PangoRectangle logical_rect;
   gint width, height;
   GSList *tmp_list;
-  GtkWidget *widget;
+  GtkWidget *widget = NULL;
 
   width = 1;
   height = 1;