Don't run handler if it doesn't exist..

libs/tornado/ioloop.py

@@ -676,6 +676,7 @@ class PollIOLoop(IOLoop):
             while self._events:
                 fd, events = self._events.popitem()
                 try:
+                    if self._handlers.has_key(fd):
                         self._handlers[fd](fd, events)
                 except (OSError, IOError) as e:
                     if e.args[0] == errno.EPIPE: