vlc_stream: wait a bit when there's no data available.
If there's no data available (vlc_stream_ReadPartial()
returns -1) we should
wait a bit for the buffers to replenish. Otherwise we might loop infinitely,
wasting CPU cycles.
If we get -1 that means the reader is not going to do any waiting for us, so we should do it here.
vlc_tick_sleep()
is canceleable so it has no impact on the cancelability of the call.