Drop reference to timer callback on clearTimeout

Reported here:
http://groups.google.com/group/nodejs-dev/browse_thread/thread/9e063d0938f99879

Would be good to test this somehow...

src/node.js

@@ -425,6 +425,7 @@ global.setInterval = function (callback, repeat) {
 global.clearTimeout = function (timer) {
   if (!Timer) Timer = process.binding("timer").Timer;
   if (timer instanceof Timer) {
+    timer.callback = null;
     timer.stop();
   }
 };