Fix perf node division by zero.

game/game.py

@@ -56,6 +56,7 @@ class PerfGroup(Group):
         self._name = name
 
     def __del__(self):
+        if self._count:
             avg = round(self._total / self._count, 2)
             print(self._name, "*", self._count,
                 ": min =", round(self._min, 2), ", max =", round(self._max, 2), ", avg =", avg, "(ms)")