Postpone clearing old MIDI events until next event

This fixes Native Instrument's FM7 crashing on MIDI input. The plugin
expects the last received MIDI event to always be alive during audio
processing, even if there have not been any new events in this
processing cycle.

src/wine-host/bridges/vst2.cpp

@@ -278,7 +278,9 @@ Vst2Bridge::Vst2Bridge(MainContext& main_context,
                         }},
                     request.buffers);
 
-                next_audio_buffer_midi_events.clear();
+                // See the docstrong on `should_clear_midi_events` for why we
+                // don't just clear `next_buffer_midi_events` here
+                should_clear_midi_events = true;
             });
     });
 }
@@ -303,6 +305,14 @@ void Vst2Bridge::run() {
                 // plugin.
                 std::lock_guard lock(next_buffer_midi_events_mutex);
 
+                // See the docstring on `should_clear_midi_events` for why we
+                // only deallocate old MIDI events here instead of a at the end
+                // of every processing cycle
+                if (should_clear_midi_events) {
+                    next_audio_buffer_midi_events.clear();
+                    should_clear_midi_events = false;
+                }
+
                 next_audio_buffer_midi_events.push_back(
                     std::get<DynamicVstEvents>(event.payload));
                 DynamicVstEvents& events = next_audio_buffer_midi_events.back();