Only process valid connections

The `_from_server_socket routine` may return a None which should
not be sent to workers to process.

for #358

cheroot/connections.py

@@ -216,7 +216,8 @@ def _run(self, expiration_interval):
  if conn is self.server:
  # New connection
  new_conn = self._from_server_socket(self.server.socket)
- self.server.process_conn(new_conn)
+ if new_conn is not None:
+ self.server.process_conn(new_conn)
  else:
  # unregister connection from the selector until the server
  # has read from it and returned it via put()