Audio/MidiJack: Fix invalid read: (3) of #5408

This atomically unsets the MidiJack reference in AudioJack right before
MidiJack is destroyed. This avoids AudioJack using a destroyed MidiJack
object.

include/AudioJack.h

@@ -58,6 +58,7 @@ class AudioJack : public QObject, public AudioDevice
 	// the jack callback is handled here, we call the midi client so that it can read
 	// it's midi data during the callback
 	AudioJack * addMidiClient(MidiJack *midiClient);
+	void removeMidiClient(void) { m_midiClient = nullptr; }
 	jack_client_t * jackClient() {return m_client;};
 
 	inline static QString name()
@@ -109,7 +110,7 @@ private slots:
 	bool m_active;
 	std::atomic<bool> m_stopped;
 
-	MidiJack *m_midiClient;
+	std::atomic<MidiJack *> m_midiClient;
 	QVector<jack_port_t *> m_outputPorts;
 	jack_default_audio_sample_t * * m_tempOutBufs;
 	surroundSampleFrame * m_outBuf;

src/core/audio/AudioJack.cpp

@@ -346,8 +346,8 @@ int AudioJack::processCallback( jack_nframes_t _nframes, void * _udata )
 	// add to the following sound processing
 	if( m_midiClient && _nframes > 0 )
 	{
-		m_midiClient->JackMidiRead(_nframes);
-		m_midiClient->JackMidiWrite(_nframes);
+		m_midiClient.load()->JackMidiRead(_nframes);
+		m_midiClient.load()->JackMidiWrite(_nframes);
 	}
 
 	for( int c = 0; c < channels(); ++c )

src/core/midi/MidiJack.cpp

@@ -116,6 +116,9 @@ MidiJack::~MidiJack()
 {
 	if(jackClient())
 	{
+		// remove ourselves first (atomically), so we will not get called again
+		m_jackAudio->removeMidiClient();
+
 		if( jack_port_unregister( jackClient(), m_input_port) != 0){
 			printf("Failed to unregister jack midi input\n");
 		}