Assertion when media transport initialization fails and NULL instead DISCONNECTED is reported in the on_call_state() callback

[bennylp]

There are two problems:

1. When media transport initialization fails, it looks like PJSUA-LIB ignores this and continue with the call, causing assertion that looks like this:

   pjsua-x86_64-unknown-linux-gnu: ../src/pjmedia/endpoint.c:770: 
   pjmedia_endpt_create_base_sdp: Assertion `!"Invalid address family"' failed.
   

2. When the above is fixed, the call gets disconnected as expected. But during disconnection callback via on_call_state() callback, call state is reported as NULL instead of DISCONNECTED.

[bennylp]

In r4134: fixed the first problem

[bennylp]

In r4135: fixed second problem

[bennylp]

In r4136: fixed the incoming call scenario, and changed disconnection code from Precondition Failure to Temporarily Unavailable