Send ACK for incoming 200/INVITE even after call is disconnected

[nanang]

Scenario:

1. A calls B
2. A sends re-INVITE and immediately sends BYE (before receiving final response for the re-INVITE)
3. A receives 200/BYE, call disconnected both sides
4. A receives 200/INVITE and the retransmissions as A doesn't send ACK.

In this case, A should send ACK to complete the re-INVITE transaction, as described in ​rfc5407 appendix-B:

   As is mentioned above, the dialog and the
   transaction behave independently each other.  Therefore, the
   transaction handling has to be continued even though the dialog has
   moved to the Terminated state.

Thanks Itay Bianco for the report.

[nanang]

In 4806:

Fix #1755: Send ACK for incoming 200/INVITE even after call is disconnected.