- return null instead of throwing exception if CD playback isn't supported

The game should switching to MIDI music on platforms without CD playback support
It was impossible to start Hexen on platforms other than Windows

libraries/zmusic/musicformats/music_cd.cpp

@@ -196,16 +196,14 @@ MusInfo* CDDA_OpenSong(MusicIO::FileInterface* reader)
 
 #else
 
-#include <stdexcept>
-
 MusInfo* CD_OpenSong(int track, int id)
 {
-	throw std::runtime_error("CD Audio playback not supported");
+	return nullptr;
 }
 
 MusInfo* CDDA_OpenSong(MusicIO::FileInterface* reader)
 {
-	throw std::runtime_error("CD Audio playback not supported");
+	return nullptr;
 }