print warning when we receive a presence from an invalid JID

src/common/connection_handlers_events.py

@@ -710,6 +710,7 @@ PresenceHelperEvent):
         try:
             self.get_jid_resource()
         except Exception:
+            log.warn('Invalid JID: %s, ignoring it' % self.stanza.getFrom())
             return
         jid_list = gajim.contacts.get_jid_list(self.conn.name)
         self.timestamp = None