combine-sink: Fix threading issue during underrun

A recent commit added i->origin sink for the sink inputs of the combine sinks. Therefore pa_sink_process_input_underruns() treated the combine sink like filter sinks. pa_sink_process_input_underruns() calls itself with the origin sink, which is only correct for filter sinks because they run in the thread context of the origin sink. The combine sink however has its own thread context, so pa_sink_process_input_underruns() was executed in the wrong context. This patch fixes the issue by skipping the section for module-combine-sink.