decoder: ignore the SEEK command during startup

While waiting for the input stream to become ready, ignore all
commands except STOP.  This fixes seeking errors with (remote) songs
which the decoder has already finished.

src/decoder_thread.c

@@ -109,7 +109,7 @@ static void decoder_run(void)
 	   will be available then */
 
 	while (!input_stream.ready) {
-		if (dc.command != DECODE_COMMAND_NONE) {
+		if (dc.command == DECODE_COMMAND_STOP) {
 			input_stream_close(&input_stream);
 			dc.state = DECODE_STATE_STOP;
 			return;