gtkprintunixdialog: Fix a potential g_object_unref(NULL) call

The code above checks whether (printer == NULL), so we’d better do so
here as well.

Found by scan-build.

https://bugzilla.gnome.org/show_bug.cgi?id=712760

gtk/gtkprintunixdialog.c

@@ -961,7 +961,7 @@ void set_cell_sensitivity_func (GtkTreeViewColumn *tree_column,
   else
     g_object_set (cell, "sensitive", TRUE, NULL);
 
-  g_object_unref (printer);
+  g_clear_object (&printer);
 }
 
 static void