Fixed ICE stagnation when connectivity check fails

[ming]

If one connectivity check fails, the checklist periodic check will not continue, hence resulting the ICE negotiation to make no progress.

2016-08-22 12:07:00.391 icetp00 Starting checklist periodic check
2016-08-22 12:07:00.393 icetp00 Sending connectivity check for check 0: [1] 192.168.100.10:56958-->192.168.100.20:51136
2016-08-22 12:07:00.394 utsx0x163a8570 STUN client transaction created
2016-08-22 12:07:00.394 utsx0x163a8570 STUN sending message (transmit count=1)
2016-08-22 12:07:00.394 utsx0x163a8570 STUN error sending message: Network is unreachable
2016-08-22 12:07:00.395 stun_session.c tdata 0x163a8464 destroy request, force=0, tsx=0x163a8570
2016-08-22 12:07:00.395 utsx0x163a8570 STUN transaction 0x163a8570 schedule destroy
2016-08-22 12:07:00.396 stuse0x15c2e00 Error sending STUN request: Network is unreachable
2016-08-22 12:07:00.397 icetp00 Error sending STUN request: Network is unreachable
...
[no ICE progress after this]

[ming]

In 5427:

Fixed #1957: ICE stagnation when connectivity check fails