DecoderControl: reduce the number of PlayerThread wakeups

Wake up the PlayerThread only if it is really waiting for the decoder.
This greatly reduces the number of system calls in the DecoderThread.

src/DecoderInternal.cxx

@@ -102,6 +102,7 @@ decoder_flush_chunk(Decoder &decoder)
 	decoder.chunk = nullptr;
 
 	dc.Lock();
-	dc.client_cond.signal();
+	if (dc.client_is_waiting)
+		dc.client_cond.signal();
 	dc.Unlock();
 }