#1873 GTK: Don't forward calls to NULL scrollbars

[Colomban Wendling]

Once Dispose() has been called, scrollbars will be NULL, so we need to check against this in code that might run after Dispose().

Fixes scary warnings on certain widget destruction scenarios:

Gtk-CRITICAL **: gtk_widget_unmap: assertion 'GTK_IS_WIDGET (widget)' failed

Gtk-CRITICAL **: gtk_widget_unmap: assertion 'GTK_IS_WIDGET (widget)' failed

Gtk-CRITICAL **: gtk_widget_unrealize: assertion 'GTK_IS_WIDGET (widget)' failed

Gtk-CRITICAL **: gtk_widget_unrealize: assertion 'GTK_IS_WIDGET (widget)' failed

[Neil Hodgson]

Committed as [4e67e6].