Handle edge case where socket is None (#686)

* 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 <[email protected]>

src/paho/mqtt/client.py

@@ -642,6 +642,10 @@ def _sock_recv(self, bufsize):
         except ssl.SSLWantWriteError:
             self._call_socket_register_write()
             raise BlockingIOError
+        except AttributeError as err:
+            self._easy_log(
+                MQTT_LOG_DEBUG, "socket was None: %s", err)
+            raise ConnectionError
 
     def _sock_send(self, buf):
         try: