Unregister request should be sent out only while registered

While trigger the registration through cli cmd 'pjsip send register {arg}', the
un-register request is sent regardless if it was registered or not. This caused
issues while it was not registered, the un-register request will fall into
recovery or retry mechanism if the un-register request sent failed, and will keep
trying with expire=0 (un-register), hence no regular register could be sent out.