Stop music, so that its handle can be freed, before playing other mus…

…ic on sector change

src/supertux/sector.cpp

@@ -1506,6 +1506,7 @@ Sector::resume_music()
   }
   else
   {
+    SoundManager::current()->stop_music();
     SoundManager::current()->play_music(music, true);
   }
 }