Fixed crash when hanging up call if call invite hasn't been created

[ming]

Program received signal SIGSEGV, Segmentation fault.

0x0000000000429047 in pjsua_call_hangup (call_id=2, code=0, reason=0x0,
    msg_data=0x0) at ../src/pjsua-lib/pjsua_call.c:2370
2370   if (call->inv->role == PJSIP_ROLE_UAS)

Step to reproduce:
Run pjsua with dummy TURN server.
--use-ice --use-turn --turn-srv 8.8.8.8:12345 --turn-user na --turn-passwd na
then make call and immediately hangup

If ICE setup takes a long time, for example when using a non-responsive TURN server, call->inv hasn't been created yet, thus causing the crash.

Analysis:
Before the crashing line, there's a conditional statement:

    if ((call->med_ch_cb && !call->inv) ||
	((call->inv != NULL) && (call->inv->state == PJSIP_INV_STATE_NULL)))

So, it's possible to enter the block with call->inv == NULL, however later we immediately access call->inv->role, thus causing the crash.

Thanks to Håkan Berg for the report.

[ming]

In 5816:

Fixed #2125: Crash when hanging up call if call invite hasn't been created