Handle edge case where socket is None

* The following exception happens randomly on startup:

```
Exception has occurred: AttributeError
'NoneType' object has no attribute 'recv'
File "paho/mqtt/client.py", line 640, in _sock_recv
```

* The root cause is unknown, but this allows the library to reconnect
  gracefully.

Signed-off-by: Robert DeRose <RobertDeRose@gmail.com>

src/paho/mqtt/client.py

@@ -2419,7 +2419,7 @@ def _packet_read(self):
                 data = self._sock_recv(self._in_packet['to_process'])
             except BlockingIOError:
                 return MQTT_ERR_AGAIN
-            except ConnectionError as err:
+            except (ConnectionError, AttributeError) as err:
                 self._easy_log(
                     MQTT_LOG_ERR, 'failed to receive on socket: %s', err)
                 return MQTT_ERR_CONN_LOST