Stop parsing after closeConnection()

- Parsing continued after closeConnection(), leaving the client in the
"connected" state with a dead socket. This led to a permanent,
unrecoverable wedge in Release builds and a crash in Debug builds.

- Check whether the client is already disconnected before changing its
state to BrokerConnected.

Pick-to: 6.11 6.12 6.8
Task-number: QTBUG-149565
Change-Id: I13d45c24d24fe862d9aec6a7e421f5ef4d494db0
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>