Fixed crash in case that the .music file does not exist.

git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/trunk/supertuxkart@1945 178a84e3-b1eb-0310-8ba1-8eac791a3b58

src/sound_manager.cpp

@@ -129,6 +129,12 @@ void SoundManager::addMusicToTracks()
     for(std::map<std::string,MusicInformation*>::iterator i=m_allMusic.begin();
                                                           i!=m_allMusic.end(); i++)
     {
+        if(!i->second) 
+        {
+            fprintf(stderr, "Can't find music file '%s' - ignored.\n",
+                    i->first.c_str());
+            continue;
+        }
         i->second->addMusicToTracks();
     }
 }   // addMusicToTracks