BYE is not sent when 200/OK is not ACK-ed

[riza]

Scenario:

1. Make call from device A to device B
2. Answer the call on device B and disconnect immediately.
3. In case device B disconnected the call (hangup) and ACK is lost, BYE will not be send and call will not be disconnected.

In case device B didn't hangup, timeout will occur and BYE is sent
(200/OK is not ACK-ed).

However, on the above case, B hangup and got delayed until ACK is received.
When the timeout occur, the stack tried to hangup but still got delayed.
This is because, the hangup cause cannot be changed due to this check on inv_set_cause

(cause_code > inv->cause)

Thanks to Nir Lavi for the report.