Initialize pointers to null on creation

Fixes access violation in hello_world.ppu.elf on window close.
As I understood the reason - when dtor of XAudioThread is called it
checks:
if (m_source_voice) Quit();
But m_source_voice isn't initialized to 0 by default so when in Quit()
in tries to call some funcs from unitialized ptrs and access violation
happens.

rpcs3/Emu/Audio/XAudio2/XAudio2Thread.cpp

@@ -11,6 +11,10 @@ XAudio2Thread::~XAudio2Thread()
 	if (m_source_voice) Quit();
 }
 
+XAudio2Thread::XAudio2Thread() : m_xaudio2_instance(nullptr), m_master_voice(nullptr), m_source_voice(nullptr)
+{
+}
+
 void XAudio2Thread::Init()
 {
 	HRESULT hr = S_OK;

rpcs3/Emu/Audio/XAudio2/XAudio2Thread.h

@@ -14,6 +14,7 @@ private:
 
 public:
 	virtual ~XAudio2Thread();
+	XAudio2Thread();
 
 	virtual void Init();
 	virtual void Quit();