Fix the IOStream close callback not getting called if there are pendi…

…ng callbacks.

_maybe_add_error_listener only does anything if _state is None.

tornado/iostream.py

@@ -224,6 +224,7 @@ def close(self):
                                    self._consume(self._read_buffer_size))
             if self._state is not None:
                 self.io_loop.remove_handler(self.socket.fileno())
+                self._state = None
             self.socket.close()
             self.socket = None
             if self._close_callback and self._pending_callbacks == 0: