ffmpeg_filter: initialize sub2video.end_pts together with last_pts

This way re-initializations properly update end_pts, enabling
sub2video_heartbeat to call sub2video_update as expected to re-init
the sub2video AVFrame's contents and to feed a frame into the filter
chain.

This then fixes memory usage ballooning due to framesync waiting
for secondary input in case of no actual subtitle samples being present
for a while in source after a re-init occurs.

fftools/ffmpeg_filter.c

@@ -739,6 +739,7 @@ static int sub2video_prepare(InputStream *ist, InputFilter *ifilter)
     if (!ist->sub2video.frame)
         return AVERROR(ENOMEM);
     ist->sub2video.last_pts = INT64_MIN;
+    ist->sub2video.end_pts  = INT64_MIN;
     return 0;
 }