core: don't callback self.conn_deferred if already called:

if the connection has been lost (e.g. forced disconnection by server),
`self.conn_deferred` may have already been called, thus this is now checked to avoid an
exception.

sat/core/xmpp.py

@@ -302,7 +302,11 @@ class SatXMPPEntity:
         self.streamInitialized()
 
     def _finish_connection(self, __):
-        self.conn_deferred.callback(None)
+        if self.conn_deferred.called:
+            # can happen in case of forced disconnection by server
+            log.debug(f"{self} has already been connected")
+        else:
+            self.conn_deferred.callback(None)
 
     def streamInitialized(self):
         """Called after _authd"""