VST3: explicitly close and release view

A call to VST3Plugin::has_editor() can create the view of
a plugin. If the GUI is never displayed, the view would
not have been released, which can lead to crashes when
unloading the plugin.

libs/ardour/vst3_plugin.cc

@@ -1106,6 +1106,8 @@ VST3PI::unit_data ()
 void
 VST3PI::terminate ()
 {
+	close_view ();
+
 	/* disable all MIDI busses */
 	set_event_bus_state (false);