decoder: additional dc.command checks in decoder_data()

When a command is sent while the decoder waits for a free chunk in the
music pipe, it was not returned by decoder_data().

src/decoder_api.c

@@ -168,6 +168,9 @@ need_chunks(struct input_stream *is, bool wait)
 	if ((is == NULL || input_stream_buffer(is) <= 0) && wait) {
 		notify_wait(&dc.notify);
 		notify_signal(&pc.notify);
+
+		if (dc.command != DECODE_COMMAND_STOP)
+			return dc.command;
 	}
 
 	return DECODE_COMMAND_NONE;