don't forget password when authentication fails. It may be a server problem. Fixes #5779

src/common/connection.py

@@ -1308,8 +1308,6 @@ class Connection(CommonConnection, ConnectionHandlers):
                 self.on_connect_auth(con)
                 self.on_connect_auth = None
         else:
-            # Forget password, it's wrong
-            self.password = None
             gajim.log.debug("Couldn't authenticate to %s" % self._hostname)
             self.disconnect(on_purpose = True)
             self.dispatch('STATUS', 'offline')