After bind to the the server web service via SoapBindingStub, the binding will live during the whole web container life cycle.
Actually ,the binding to the server will be invalid because the session can be timeout. So if the bind is invalid, we should bind
to the server again.
Even if the bind is invalid, the bind is not null. We can't determine the The binding validation by checking the bind null or not.
1.Invalid sessionid found in session header: session timedout
2. Salesforce Web Service API Developer's Guide