Fix warning due to wrong use of NULL in wxOSX wxSound.

This is the backport of the old r76862 from trunk.

src/osx/core/sound.cpp

@@ -51,7 +51,7 @@ protected:
 };
 
 wxOSXAudioToolboxSoundData::wxOSXAudioToolboxSoundData(const wxString& fileName) :
-    m_soundID(NULL)
+    m_soundID(0)
 {
     m_sndname = fileName;
     m_playing = false;