Clear level stats vector before loading them into the vector

Otherwise this results in what is basically a memory leak that can be
triggered repeatedly by selecting "play a level" over and over again.

Fixes #885.

desktop_version/src/Game.cpp

@@ -483,6 +483,8 @@ void Game::loadcustomlevelstats(void)
         return;
     }
 
+    customlevelstats.clear();
+
     // Old system
     std::vector<std::string> customlevelnames;
     std::vector<int> customlevelscores;