Don't kill the NiceDataLoop when no packets are available.

It's entirely possible that the timed_wait in NiceConnection::getPacket
fires before we get any data on the NiceConnection. When that happens,
we don't want to kill the thread passing data on to the DtlsTransport.
Merge pull request #109 from byronclark/dtls-nice-startup-race

Don't kill the NiceDataLoop when no packets are available.