on_call_transfer_status() callback is not called when REFER is responded with failure response

[riza]

Scenario:

A                      B
<-- Established call -->
--------  REFER   ----->
<-------  4xx     ------ 

Currently, the log will print:
"Warning: received NOTIFY without message body"
and the on_call_transfer_status() callback is not called.

Thanks to Joshua Elliott for the report.

[riza]

In 5710:

Close #2072: Call on_call_transfer_status() callback when REFER is responded with failure response.