Missing transaction/cseq check for incoming ACK in an INVITE session (thanks Ilya Kalinin for the report)

[nanang]

Case scenario:

1. Receiving normal INVITE with SDP, processed normally until reaching confirmed state, let's call this TSX 1.
2. Receiving re-INVITE without SDP, responded 200 OK with SDP, and waiting for an ACK with SDP, let's call this TSX 2.
3. Receiving ACK retransmission of TSX 1, problem start here: it is processed as if it is for TSX 2, as there is no SDP, the local SDP offer of TSX 2 gets cancelled, so there is no media session update.
4. Receiving ACK with SDP for TSX 2, the SDP is ignored as SDP negotiator is in state DONE.