Fix on_disconnect() using wrong callback.

It would sometimes call the MQTT v3.x callback when it should
call the MQTT v5 callback.

Closes #570. Thanks to Frank Pagliughi.

ChangeLog.txt

@@ -21,6 +21,8 @@ v1.6.0 - 2021-xx-xx
   _out_packet queue use to thread safe.
 - Add basic MQTT v5 support to the subscribe and publish helper functions.
   Closes #575.
+- Fix on_disconnect() sometimes calling the MQTT v3.x callback when it should
+  call the MQTT v5 callback. Closes #570.
 
 
 v1.5.1 - 2020-09-22

src/paho/mqtt/client.py

@@ -3413,7 +3413,7 @@ class Client(object):
         if on_disconnect:
             with self._in_callback_mutex:
                 try:
-                    if properties:
+                    if self._protocol == MQTTv5:
                         on_disconnect(
                             self, self._userdata, rc, properties)
                     else: