Unregistration does not respond to authentication challenge (401/407) after it has been marked for deletion

[bennylp]

After application has marked client registration session for deletion(pjsip_regc_destroy() is called), the client registration session does not respond to authentication challenge in 401/407 response.

Here are the steps to reproduce it:

 // Create Un-REGISTER request
 status = pjsip_regc_unregister(regc, &tdata);

 // Send the request.
 status = pjsip_regc_send(regc, tdata);

 // Signal regc to destroy itself after all transactions complete
 status = pjsip_regc_destroy(regc);  

[bennylp]

Fixed in r891.