nicesink: unblock pending write on component change

When a nicesink switches to a different component, we should unblock any
pending write waiting in gst_nice_sink_render() for writable_cond being
signalled because if the new transport already is writable, we'll never
get notified about the condition and thus nicesink will wait forever.

That way, either the pending data are immediately sent or the nicesink
blocks again until the transport becomes writable.
agent: ignore externally set peer-reflexive candidates

All discovered peer-reflexive candidates should be added internally in
conncheck.c and should have the sockptr set. We ignore any prflx
candidates added by an external application because their NULL sockptr
could cause a crash in conn_check_send().