Fixed #1978 (by removing unnecessary stopMusic in World, which actually

stops the menu music, not the tutorial music. It's unnecessary since
startMusic (main menu) later will first stop currently running music.

src/modes/world.cpp

@@ -404,7 +404,6 @@ World::~World()
     if(m_physics)
         delete m_physics;
 
-    music_manager->stopMusic();
     m_world = NULL;
 
     irr_driver->getSceneManager()->clear();