PYTHON-898 - Fix case of getmore with removed RS member.

Author: ajdavis

pymongo/topology.py

@@ -308,7 +308,8 @@ def _apply_selector(self, selector, address):
  # Ignore the selector.
  return self._description.known_servers
  elif address:
- return [self._description.server_descriptions().get(address)]
+ sd = self._description.server_descriptions().get(address)
+ return [sd] if sd else []
  elif self._description.topology_type == TOPOLOGY_TYPE.Sharded:
  return apply_local_threshold(self._settings.local_threshold_ms,
  self._description.known_servers)