Allow cutscenes to have no music, a few minor other fixes in animation

git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/trunk@11315 178a84e3-b1eb-0310-8ba1-8eac791a3b58

src/audio/music_manager.hpp

@@ -78,6 +78,7 @@ public:
     void                    loadMusicFromOneDir(const std::string& dir);
     void                    addMusicToTracks();
 
+    void                    clearCurrentMusic() { m_current_music = NULL; }
 };
 
 extern MusicManager* music_manager;

src/tracks/track.cpp

@@ -390,6 +390,8 @@ void Track::startMusic() const
     // In case that the music wasn't found (a warning was already printed)
     if(m_music.size()>0)
         music_manager->startMusic(m_music[rand()% m_music.size()], false);
+    else
+        music_manager->clearCurrentMusic();
 }   // startMusic
 
 //-----------------------------------------------------------------------------