audio: ensure candidate pairs in sipe_backend_get_local_candidates
Sometimes purple will not return complete list of candidates, even
after "candidates-prepared" signal is emitted. This is a feature of
libnice, namely affecting candidates discovered via UPnP. Nice does
not wait until discovery is finished and can signal end of candidate
gathering before all responses from UPnP enabled gateways are received.
Remove any incomplete RTP+RTCP candidate pairs from the list.
For details see this thread on libnice mailing list
http://lists.freedesktop.org/archives/nice/2010-July/000317.html