Add index.html

http/http_ffmpeg.c

@@ -142,7 +142,7 @@ static void http_ffmpeg_video(http_worker_t *worker, FILE *stream, const char *c
     return;
   }
 
-  http_500_header(stream);
+  http_500(stream, NULL);
 
   if (n == 0) {
     fprintf(stream, "No frames.\n");