Assertion when SDP negotiation processing SDP offer containing disabled media without rtpmap attribute

[nanang]

Here is the scenario to reproduce:

1. Receiving offer with one media line disabled and no rtpmap for dynamic PT (valid):

   m=audio 5000 RTP/AVP 0
   m=video 0 RTP/AVP 100
   

2. PJSUA answers similarly (no rtpmap for dynamic PT), e.g:

   m=audio 4000 RTP/AVP 0
   a=rtcp:4001 IN IP4 192.168.1.30
   a=rtpmap:0 PCMU/8000
   a=sendrecv
   m=video 0 RTP/AVP 100
   

3. PJSUA sends re-INVITE (using latest active local SDP as above), after receiving the answer, assertion raised in SDP negotiation.

The SIPp scenario can be found in tests/pjsua/scripts-sipp/uac-inv-two-media-but-one-disabled-no-rtpmap.xml.

[nanang]

(In [3192]) Fix #1088:

• Fixed SDP negotiation to ignore disabled media (with port 0) in the answer.
• Added a SIPp scenario for reproducing the issue.