Assertion when receiving INVITE with no SDP and video is deactivated (thanks Bogdan Krakowski for the report)

[nanang]

Reproducing steps with pjsua:

1. build pjsua with video feature
2. run the pjsua without --video param, so video is deactivated
3. send INVITE to pjsua without SDP
4. answer the call, e.g: "a" then "200", there will be video in the SDP (which should not!)
5. send ACK to pjsua with SDP answer with video in the answer is enabled too

After investigation, after receiving incoming call, PJSUA will immediately start media channel init and create SDP with default call setting, i.e: audio and video count both set to 1, and when application call pjsua_call_answer() with different call setting, e.g: video count set to 0, the media channel update doesn't like such inconsistency when we are as SDP offerer and will trigger an assertion.

The solution should consider the requirements below:

• be able to initialize media with the settings based on user confirmation
• should avoid excessive/repeated initialization (e.g: multiple SDP or transport creation)
• simple and minimal changes is prefered