Only reset the self-ping event timer if there was already an enabled timer

Otherwise we could, for example, enable a self-ping event on non-joined
rooms where we just displayed an error message. And that’s bad.

src/tabs/muctab.py

@@ -1618,7 +1618,8 @@ class MucTab(ChatTab):
         """
 
         # reset self-ping interval
-        self.enable_self_ping_event()
+        if self.self_ping_event:
+            self.enable_self_ping_event()
 
         self.log_message(txt, nickname, time=time, typ=kwargs.get('typ', 1))
         args = dict()