Fix possible AttribueError during conn._close_socket (#1776)

Author: dpkp

kafka/conn.py

@@ -769,7 +769,7 @@ def _update_reconnect_backoff(self):
             log.debug('%s: reconnect backoff %s after %s failures', self, self._reconnect_backoff, self._failures)
 
     def _close_socket(self):
-        if self._sock:
+        if hasattr(self, '_sock') and self._sock is not None:
             self._sock.close()
             self._sock = None