core: Don't crash if a stream is missing a side.

mitogen/core.py

@@ -619,7 +619,9 @@ class BasicStream(object):
         LOG.debug('%r.on_disconnect()', self)
         broker.stop_receive(self)
         broker.stop_transmit(self)
+        if self.receive_side:
             self.receive_side.close()
+        if self.transmit_side:
             self.transmit_side.close()
         fire(self, 'disconnect')