dde_bsd: audio do not advance on silence

Do not advance ring buffer positions when playing silence, additionally
to not send progress signals when playing silence. Silence implies
underrun in the client side - not progress.

issue #4609

repos/dde_bsd/src/drivers/audio/main.cc

@@ -142,6 +142,7 @@ class Audio_out::Out
 
 			} else {
 				_play_silence();
+				return;
 			}
 
 			_advance_position(p_left, p_right);