ffmpeg: init sub2video.last_pts.

Get the heartbeat working when the video has negative timestamps.

Fix trac ticket #4062.
Signed-off-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg_filter.c

@@ -619,6 +619,7 @@ static int sub2video_prepare(InputStream *ist)
     ist->sub2video.frame = av_frame_alloc();
     if (!ist->sub2video.frame)
         return AVERROR(ENOMEM);
+    ist->sub2video.last_pts = INT64_MIN;
     return 0;
 }