inspector: Avoid a NULL deref

Coverity pointed out that getting a class name requires
dereferencing the pointer, so only don't do it if it is NULL.

gtk/inspector/widget-tree.c

@@ -283,7 +283,10 @@ gtk_inspector_widget_tree_append_object (GtkInspectorWidgetTree *wt,
   else
     mapped = TRUE;
 
-  class_name = G_OBJECT_CLASS_NAME (G_OBJECT_GET_CLASS (object));
+  if (G_IS_OBJECT (object))
+    class_name = G_OBJECT_CLASS_NAME (G_OBJECT_GET_CLASS (object));
+  else
+    class_name = "";
 
   if (GTK_IS_WIDGET (object))
     {