ffmpeg: Add sanity check on frame duplication

This prevents long loops and OOM in case of damaged timestamps in some
cases
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -1770,6 +1770,11 @@ static void do_video_out(AVFormatContext *s,
         av_log(NULL, AV_LOG_VERBOSE, "*** drop!\n");
         return;
     } else if (nb_frames > 1) {
+        if (nb_frames > dts_error_threshold * 30) {
+            av_log(NULL, AV_LOG_ERROR, "%d frame duplication too large, skiping\n", nb_frames - 1);
+            nb_frames_drop++;
+            return;
+        }
         nb_frames_dup += nb_frames - 1;
         av_log(NULL, AV_LOG_VERBOSE, "*** %d dup!\n", nb_frames - 1);
     }