Unregistration does not respond to authentication challenge (401/407) in some cases

[bennylp]

When unregistration is performed while application has released its registration session (pjsip_regc_destroy() is called), the client registration does not respond to authentication challenge in 401/407 response.

Here's 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);  

The bug occurs because when the _delete_flag is set internally because of pjsip_regc_destroy(), the processing will stop immediately rather than processing the 401/407 response.