transcode: Fix input stream check

transcode.c

@@ -318,7 +318,7 @@ int main(int argc, char *argv[])
 	/* Flush */
 	for (i = 0; i < demux->nb_streams; i ++)
 	{
-		if (decs[i])
+		if (!decs[i] || !srcs[i])
 			continue;
 		filtergraph_send(srcs[i], decs[i], 0);
 		!av_buffersrc_add_frame(srcs[i], 0);