ffmpeg: fix massive leak occurring when seeking

Avoid to add frames to the vsrc_buffer in the case ist->pts <
start_time, as these frames are unused (and never released). In
particular this condition is verified with commands of the kind:
ffmpeg -i INPUT -ss TIME OUTPUT

Also allow a minor simplification.

ffmpeg.c

@@ -1656,21 +1656,6 @@ static int output_packet(AVInputStream *ist, int ist_index,
             avpkt.size = 0;
         }
 
-#if CONFIG_AVFILTER
-        if(ist->st->codec->codec_type == AVMEDIA_TYPE_VIDEO) {
-            for(i=0;i<nb_ostreams;i++) {
-                ost = ost_table[i];
-                if (ost->input_video_filter && ost->source_index == ist_index) {
-                    if (!picture.sample_aspect_ratio.num)
-                        picture.sample_aspect_ratio = ist->st->sample_aspect_ratio;
-                    picture.pts = ist->pts;
-
-                    av_vsrc_buffer_add_frame(ost->input_video_filter, &picture);
-                }
-            }
-        }
-#endif
-
         // preprocess audio (volume)
         if (ist->st->codec->codec_type == AVMEDIA_TYPE_AUDIO) {
             if (audio_volume != 256) {
@@ -1701,6 +1686,13 @@ static int output_packet(AVInputStream *ist, int ist_index,
                 ost = ost_table[i];
                 if (ost->source_index == ist_index) {
 #if CONFIG_AVFILTER
+                if (ost->input_video_filter) {
+                    if (!picture.sample_aspect_ratio.num)
+                         picture.sample_aspect_ratio = ist->st->sample_aspect_ratio;
+                    picture.pts = ist->pts;
+
+                    av_vsrc_buffer_add_frame(ost->input_video_filter, &picture);
+                }
                 frame_available = ist->st->codec->codec_type != AVMEDIA_TYPE_VIDEO ||
                     !ost->output_video_filter || avfilter_poll_frame(ost->output_video_filter->inputs[0]);
                 while (frame_available) {