Fix MidiJack crash on exit

* Fix uninitialized m_jackClient being used in MidiJack destructor
* Fix destruction order in Mixer.cpp so that MidiJack doesn't access the
  deleted AudioJack instance

Fixes #4688

src/core/Mixer.cpp

@@ -186,8 +186,8 @@ Mixer::~Mixer()
 	}
 	delete m_fifo;
 
-	delete m_audioDev;
 	delete m_midiClient;
+	delete m_audioDev;
 
 	for( int i = 0; i < 3; i++ )
 	{

src/core/midi/MidiJack.cpp

@@ -61,6 +61,7 @@ static void JackMidiShutdown(void *arg)
 
 MidiJack::MidiJack() :
 	MidiClientRaw(),
+	m_jackClient( nullptr ),
 	m_input_port( NULL ),
 	m_output_port( NULL ),
 	m_quit( false )