Fix potential bug when showing plugin-menu in different contexts

This fixes `gtk_menu_attach_to_widget(): menu already attached`
after first showing the context menu in a mixer's processor
box, and later in a different context (regionFX, I/O plugins,
route properties,..) in optimized builds.

gtk2_ardour/plugin_selector.cc

@@ -914,6 +914,9 @@ struct PluginMenuCompareByCreator {
 Gtk::Menu*
 PluginSelector::plugin_menu()
 {
+	if (_plugin_menu->get_attach_widget ()) {
+		_plugin_menu->detach ();
+	}
 	return _plugin_menu;
 }