null-sink, pipe-sink: some state variable cleanups

null-sink, pipe-sink: some state variable cleanups

pa_sink_get_state() is supposed to be used from the main thread. In this
case it doesn't really matter, because the SET_STATE handler is executed
while the main thread is waiting, but since the state is available also
in thread_info, let's use that. All other modules use thread_info.state
too, so at least this change improves consistency.

Also, we can use the PA_SINK_IS_OPENED macro to simplify the code a bit.